If you want to go the Pro Tools route, I was checking the Detroit craigslist and they have a few Digidesign 003 and 002r for sale in the $300-600 range. They come with 4 mic/line inputs and an additional 4 line inputs, I believe. Don't quote me on specs, I'm going by memory here. You should be able to bargain with the seller and get it a little cheaper. Try using eBay as a gauge for price.
If you don't need that many inputs, some guy is selling an Mbox 2 Pro (2 mic/line inputs, 2 line inputs) for $100.
You'll need to buy mics and possibly mic pres on top of that, depending on how many channels you want to record at the same time.
Have you checked out www.gearslutz.com?
Lots of good recording info on there.
Best of luck!
Edit: Oh, I see you already have an interface...In that case, get some SM 57s and/or 58s and call it a day! They go for about $100 new and there are a few on craigslist for about $70. They can serve double duty as both live and recording mics.