This page has some suggestions:
http://www.musiconmypc.co.uk/art_record_guitar_on_pc.php

Check out the Behringer interface, that's the simplest type.

I use a MAC, it's mostly preference. I bought Pro Tools LE for about $300 bucks but there are plenty of options out there. Many pedal boards will go USB straight into your PC or MAC these days.

I'm not sure about going back out to your amp, I think the tone would suffer a little bit, but it's worth a try!