If possible and affordable you'll want to look for the highest speed drive and the biggest cache possible, and of course it needs to be SATA. The true throughput of a drive is limited more by the size of the cache than the speed. Most redundant data (that which is accessed a lot) is put into the cache, and the bigger the cache the faster the response will be, unless you're using it to load extremely large files like video that will overwhelm the cache, then you might want to favor rotational speed over cache size.
It really depends on how much you're willing to spend. Hard drives, even 2.5 in units are cheap these days.