The biggest thing about hard drives to worry about, that they're likely to actually tell you, is rotation speed. Like the number of "X" on a DVD/BluRay drive, rotation speed is a measure of how fast it can spin while being read. 7200RPM is considered the standard nowadays, but you can get higher. There's also some controversy as to which brand is wonderful and which makes piles of crap masquerading as electronics; I'd say it doesn't matter if they're both popular enough to stay in the top five, but ... well, witness how big Hewlett-Packard is.
Also seconding the two-drives-with-RAID suggestion, as long as you're going as high as a terabyte.