Jon, KJ4SKP,
Good for you for asking questions!
And, kudos to Dave, KH6AQ, who gave you some wonderful info here!
Maybe I can add something even more on-point (and maybe even something that you should commit to memory, and not look to the internet for?):
1) First the really short answer:
95%+ of the time:
Use 40m daytime, with horizontal dipoles (strung up ~ 30' high) at each of your locations!
Use 80m nighttime, with horizontal dipoles (strung up ~ 50 - 60' high) at each of your locations!
(some rare times you'll need to be flexible in regards to mother nature....such as in the depths of winter, in low sunspot years, 160m might be needed at nighttime....and, in the heights of summer, especially in high sunspot years, in addition to high-summertime noise levels (lightning static crashes in SE US can be quite high), 80m absorption (D-layer) can be high and you may find your self switching bands sometimes)
Do NOT use vertical antennas here!
2) Now, the longer answers:
"Regional" radio comms over the "mid-ranges", between 20-30-40 miles and out to 300 - 500 miles, requires (and has since the days of Marconi, hi hi) using some smarts....some understanding of radiowave propagation and antenna system design / antenna patterns (and, if using MF, the ground conductivity becomes are larger part of the calculations....but no need to go there here).
3) Without rambling on and on about "ground conductivity", "NVIS", "amplifiers", "low dipoles" [sic], etc....I'd like to point out some hard facts (the "Physics") of radio comms:
a) the range you wish to cover (~ 195 miles from your house to downtown Nashville, TN....over some hilly terrain) is NOT possible to cover via "groundwave" on any MF or HF ham band....not even close!
b) the range you wish to cover (~ 195 miles from your house to downtown Nashville, TN....over some hilly terrain) is not possible (or VERY unlikely, except for some rare back-scatter or very short sporadic-E skip) to cover via "skywave" on any HF ham band above 10mhz(30m)!
c) the range you wish to cover (~ 195 miles from your house to downtown Nashville, TN....over some hilly terrain)
is most suited for Near Vertical Incidence Skywave (NVIS) communications, and this is done on the lower HF bands 40m, 60m, 80m, and 160m. (although, assuming high gain antennas and high transmit powers, 6m SSB or 2m SSB, could also be fairly reliable along this path....the costs and complexity involved, not to mention the fairly point-to-point directionality of this vhf-ssb solution, puts it way, way down the list of recommendations, and I might regret even mentioning it)
d) Near Vertical Incidence Skywave (NVIS) is, just as the name implies, an ionospheric skywave path where the signals travel "nearly vertical", and are below the ionosphere's current "critical frequency" so they are refracted back down to earth at similar / reciprocal angle.
Jon, please make a mental note here of all the above....all four of these points (facts) here, are in the ARRL Handbook, etc....and all four should be known to pass a General Class exam....so, if you commit them to memory and then go and buy an ARRL Handbook, you'll be on your way to learning / understanding a lot! 
Now, some things that might not be in the "Handbook" and/or might not be needed to pass a General Class exam.
e) As I just wrote, Near Vertical Incidence Skywave (NVIS) is an ionospheric skywave path where the signals travel "nearly vertical", and are below the ionosphere's current "critical frequency" so they are refracted back down to earth at similar / reciprocal angle.
--- Theoretically NVIS comms can occur at freqs from just below 1mhz and on up thru 12mhz (and military does use NVIS beyond the freq ranges that hams do)....but in our world of ham radio, NVIS is a propagation path that is most useful on 80m, 60m, and 40m....as well as less-so on 160m (both due to the critical freq rarely falling below 3.5mhz, and due to antenna sizes and noise levels)
--- The terms "near", "near vertical", "nearly vertical" do not have "carved-in-stone" / scientific definitions, but rather we have an "acceptance" among most RF / Antenna engineers and propagation scientists, that these generally refer to angles of ~ 50 - 60 degrees up to 90 degrees....(which, depending on ionosphere height, represent ranges from 10 miles out to ~ 250 - 300 miles....although, many hams [including myself] use "NVIS" to describe ranges out to ~ 350 - 400 miles, where the propagation angle would be ~ 45 degrees)
--- The term "critical frequency" is a scientific term --- the frequency below which, the ionosphere currently supports reflection/refraction at a vertical angle / straight-up-and-down....and, this "critical freq" is usually 1/3 of the current calculated MUF at the same location (although, this last point, being 1/3 of the MUF is rather moot, as you will almost never know the MUF at your location and along this short path, not to mention the D-layer absorption can be quite high on the lower HF bands when the MUF is high, etc.)
4) Some specific answers to your specific query, for your desired comms to/from your house and Nashville, TN....although, not knowing the space each of you have / what trees or other supports you each may have to string up an antenna on, etc...it makes it difficult to be precise in recommendations, but here goes:
As I wrote above....for 95%+ of the time:
--- Use 40m daytime, with horizontal dipoles (strung up ~ 30' high) at each of your locations!
--- Use 80m nighttime, with horizontal dipoles (strung up ~ 50 - 60' high) at each of your locations!
--- Do NOT try to use a vertical antenna for this path! It is the worst choice here! (especially a very poor / inefficient 15' long trap vertical)
---- And, while many will say that at these low heights, dipoles are omni-directional so their orientation is moot, this is actually not the case...
You see, if looking at just 2-dimensions of the azimuthal pattern you may not see a deep null off the ends...but, if you look at the elevation pattern (of say a 40m dipole at ~ 35' high), you will quickly see that while the "broadside" pattern is quite wide and allows you to even have a signal less than 1-db down at angles as low as 45 degrees, the "end-wise" pattern is already 2-db down at ~ 60 degrees and ~ 5db down at 45 degrees....and, since your path to Nashville is going to be a NVIS path at ~ 60 degree elevation (at a true bearing of ~ 318 degrees).

So, orienting your dipoles in a NE-SW directions, placing your broadside pattern in the NW-SE directions, could give you 4db of gain versus having the dipoles strung up the opposite way...
---- Also, while there are some hams that regurgitate the BS / myths that NVIS comms are better when you use "lower" antennas....this is a ridiculous myth, that would apply only when "lower" means lower than 0.4 wavelengths, but higher than 0.15 wavelengths!
This BS comes from the ground losses of very low dipoles reducing the noise levels AND the signals as well (just like their radio's receive attentuator and RF Gain controls do!)

I could add more graphs and images here to prove all of this, but if you simply accept this as fact, and understand that as dipole height gets below 0.2 to 0.175 wavelengths high, losses go up....and at really low heights of 0.02 wavelengths (about 3' on 40m, ~6' on 80m) loss is ~ -12db versus a dipole at 0.2 wavelengths!
---- In your area (and your friend's area), having below average ground conductivity, the "optimal" heights for maximum zenith gain (straight-up) is about 27' to 28' high for 40m (and 46' - 49' for 80m)....and, while maybe only adding one db or so, to improve your dipole pattern at angles of approx 60 degrees, the optimal height for your locations, for a 40m dipole, would be about 31' to 34' high (and 56' - 62' for 80m)....and, only losing one-tenth to two-tenths of a db max (.1 to .2db) "straight-up", compared to using the zenith optimal height.
---- So, you'd each need horizontal antennas for 80m and 40m
(a fan-dipole is cheap and easy to make, and a 40m/80m fan-dipole is easy-peasy to construct and tune)....40m dipole wires up 27' - 35' high (32' - 33' is best here) and 80m dipole wires up 50 - 60' high, would be great! Simple coax feed (with a good 1:1 current balun at the feedpoint), efficient, good swr bandwidth, etc...
If you (and/or your friend) don't have the room for an 80m antenna, you can make one fit in smaller space by bending it / zig-zaging it, etc....and even letting the 10' - 15' at each end hang down, etc. is just fine....(or if you wish to accept some loss and narrow-swr bandwidth, some will just add coils / loading to the dipole)
Of course, if all you want is daytime comms between the two of you....your present 40m dipole should be good (get it up ~ 30' high), just get your friend to build a simple 40m dipole and string it up ~ 30' high, and you're good-to-go!

5) So...you see....the short answer is a really good answer!
And, while the long answer is more informative, it doesn't change the basic answer....which is:
95%+ of the time:
Use 40m daytime, with horizontal dipoles (strung up ~ 30' high) at each of your locations!
Use 80m nighttime, with horizontal dipoles (strung up ~ 50 - 60' high) at each of your locations!
Do NOT use vertical antennas here!
(some rare times you'll need to be flexible in regards to mother nature....such as in the depths of winter, in low sunspot years, 160m might be needed at nighttime....and, in the heights of summer, especially in high sunspot years, in addition to high-summertime noise levels (lightning static crashes in SE US can be quite high), 80m absorption (D-layer) can be high and you may find your self switching bands sometimes)
6) BTW, your question is about propagation (and antennas), band choice, etc....but you also generically asked about "readability"....and, here you hit upon the very basics of HF comms....it's all about S/N (signal to noise ratio)!
Anything you can do to improve your S/N will help....and, while we've all been discussing way to improve each of your signals, we've ignored the "noise" part of that ratio....

Not to worry....the ARRL Handbook (and "antennas", "elmers", and "RFI" forums here on eham) will help you here!

But, in a nutshell:
a) rid your house of any/all RFI producing devices! (sometimes even the power supply running your rig can be a problem)
b) get your antennas as far away from any RFI producing devices as possible (whether in your house, or your neighbors)
c) do NOT use a "pre-amp" on the HF bands, at least not below 21mhz! (make sure both of you have your radios' pre-amps OFF!)
d) do NOT use the "noise blanker"! (except in cases where you actually do have some impulse noise or powerline noise)
e) turn down the RF Gain (and/or use the attenuator) to improve readability
f) try not to use "noise reduction"...as most "noise reduction" adds some "watery" DSP artifacts and can actually reduce "readability"
g) use headphones!!! (good communications headphones, no need for expensive "noise cancelling" headphones)
{remember that the human brain, and human ear-brain-connection, is one of the best "signal processing" devices on-the-planet! and, the more you use it, the better it gets....it is one of the advantages of actual human intelligence! Don't try to use a computer (like the DSP in a radio) to do what the human brain can do better!} h) use common-mode chokes / current baluns on your feedlines (and not using EFHW, etc. antennas)
i) use multiple turns thru some Mix 31 torroids on any wiring coming from RFI producing devices that you cannot throw away.
There are other things to do....but, think this should keep you going for now.
So, in addition to the advice about what bands, what antennas, what books to buy, etc....I hope these additional pieces of advice will help you out?I hope this helps.
73,
John, KA4WJA
{P.S. "Groudwave" and "NVIS" are unfortunately misunderstood by many hams (personally I blame the "internet", hi hi )....so, just a very quick, fyi...although use of vertical antennas (with good ground systems, and across regions with good ground conductivity) on 160m can yield good "goundwave" comms out to 80 - 100 miles....if coverage area has good-to-excellent ground conductivity, and noise levels are low (and can be 200-300 miles over a seawater path...and, I've done this myself, ~ 250 miles comms on 2mhz maritime over seawater, using vertical antennas...)
but, while the "books" say 80m "groundwave" range using the same good-ground systems and over regions with "good" to "very-good" ground conductivity, can be 50 - 60 miles, in actual real-world practice 80m "groundwave" comms is much less, typically only 30 - 40 miles, as the NVIS signal (even from the null off a vertical antenna) can overtake the weaker groundwave signal, and as noise levels are usually high for most hams these days, but if you're in areas with very low noise levels (and very good ground conductivity), you may be able to obtain the "book" ranges....(fyi, years ago I personally did some experimenting, using a random-length vertical sloper, with only one elevated radial....and my friend, just 16 miles away, using his GAP Voyager vertical....we personally found NVIS comms using our full-wave horizontal loop antennas at 35' - 50' high, produced higher signals than the groundwave signals from the verticals....real-world, for a range of just 16 miles!...not scientific, but just an fyi...)
and on 40m, this is even a shorter range (figure only ~ 20 miles for S-9 signals, but twice that if you have low noise levels and can maintain comms with low signal levels of ~ S-3 or S-4).....
As you see, "groudwave" on the ham bands is not all that useful for most....of course, the "exceptions" here are if the region covered has good-very-good ground conductivity, and the stations in question both have very-low noise levels and sustain comms even with low receive signal levels, then the ranges quoted in the "books" might be usable...and, 160m groundwave comms is done by some hams everyday, and some even use 80m groundwave, if there is poor 2m FM coverage, etc...}