Assuming you have a typical, fairly modern PC, you should look for an ATA-66 or ATA-100 IDE drive. The only real variables are capacity, speed, and manufacturer. You've already got a capacity in mind. As for speed, most drives today are 5400RPM or 7200RPM. 5400 is slower but runs cooler, 7200 is faster but runs hotter. You'll probably be happy with either one; but if you get a 7200RPM drive make sure to check that it's not too hot after a few hours of operation (it should be warm to the touch, but not painfully hot - like an AR after the first mag or two :) ). I wouldn't go faster than 7200RPM unless you are ready to buy extra fans.
There is just about as much contention about hard drive companies as about AR-type rifle companies :). There is no clear best choice. Everyone has stories about company X's drives failing after two weeks, or running fine for ten years... That said, my personal experience has been that Seagate and IBM drives tend to fail suddenly, and that Maxtor and Western Digital drives tend to run fine. (but I'm sure you'd be able to find someone who says the exact opposite =)
I wouldn't buy used. A hard drive has a limited useful life (say anywhere from 2-5 years), after which it will eventually fail. They are so cheap anyway you might as well buy new - the other day I got a 40GB 5400RPM drive for my mom for $70. (it was a Western Digital, from www.mwave.com).