I recently drove cross-country using APRS with a 50-watt rig (Yaesu FT-1500M) and a Comet SBB5NMO antenna. That Comet antenna has a gain of 5.5dBi, and I had no problems getting into a digipeater with an igate for most of the trip. I ran 25 watts from Chicago to Omaha with no problem. Thinking that the digi's would be spread further apart as I went west, I switched the rig to run 50 watts from Omaha to LA. I couldn't hit digipeaters with igates in parts of Nebraska and Colorado, but that was it. (Check my road trip's coverage map at http://www.w9tam.com/coachella-track.png
if you're curious.)
Using an HT, you're definitely going to want an antenna with some gain.