Some other possibilities for relatively narrow band sampling such as feeding a sound card:http://www.expandedspectrumsystems.com/prod2.html
I have used the Time Machine board with both the PowerSDR and SDRadio programs with good results; it provides the I/Q outputs than many of the sound card based "software radio" programs need.http://www.bright.net/~kanga/kanga/KK7B/r2pro.htm
I have seen this DC receiver used as an RF front end for DSP radios in several different places. I have not tried one myself so don't have any personal experience with it but it should be pretty easy to use.http://www.ewjt.com/kd5tfd/
You can find the Softrock40 and V5 kit info at this site. This is a rather active project with lots of buzz. Definitelty a way to dip a toe into the world of SDR's without spending much money.
Also keep in mind you can use most of this hardware and any of the software that works with them as an IF stage to an existing radio. I have done exactly that using the Time Machine card feeding either PowerSDR or SDRadio software along with a Ten Tec Corsair II. The results were very intriguing; the Corsair is an easy radio to experiment with without having to make any permanent non-reversible modifications. Many other radios of that vintage or older may also lend themselves to the same basic idea with just a few relatively minor HW modifications.