Well, Kyle was mostly right. If you're looking for something that is even less expensive and you have to get an external drive, you can go with something under 40GB (which for most people is more than you'll ever need) and a lower RPM (5400 vs. 7200 (standard)). But as Kyle said, if you're doing demanding stuff with it, more rpm = faster read = quicker load (Proc/RAM/ATA/interface dependant). Also, a great site that has stuff for way under retail is
www.pricewatch.com. They sell just about everything there, and I'm sure you can at least get a start.