FLDigi was my favorite but I haven't done any digital work in years so I don't know what's currently out there for software.
As far as an interface, it can be as simple as two patch cords. It's not an ideal set up but I did it that way for a while. Route radio audio out into the sound card mic in, and sound card audio out into radio mic in. Technically the rig and sound card should be isolated to prevent ground loops, RFI etc., but I had no problems with that at QRP. You'll also have to key the transmitter manually - I used a foot operated switch but a TX button on trhe rig front panel would also work. I experimented with VOX to key the transmitter but it wasn't very reliable.
The one major disadvantage to this method is that if you use the rig or computer for anything else and change the audio settings in either or both of them, you'll have to go back in a re-apply all the settings each time you run digital modes.