I'd go with either a
74gb SATA WD Raptor, which is going for $160 AR on newegg, or a
Seagate 200gb SATA Barracuda, which is going for $130 on newegg.
The 74gb Raptor would give you the best speed, but at the cost of less storage space and more $$. It comes in at 10000rpm and has an 8mb cache, as well as a 5-year warranty. Seek time = 4.5ms.
The 200gb Barracuda gives you more storage and saves you $$, but it isn't as fast as the Raptor. If you do a lot of movies, music, or other large files, this would be a good drive. It's still pretty quick at 7200rpm, and it has an 8mb cache, plus it's pretty quiet (although I've heard that the Raptor isn't too loud either). The Barracuda also has a 5-year warranty. Seek time = 8.5ms.
Or you could get both - the Raptor as your main speedy-fast hard drive and the Barracuda as your monster-big storage drive.
Just ask yourself what you want - speed or storage space.