If you're crazy and want more speed, get an IDE RAID controller (some mobos have them built in) and 4 of those spiffy western digital special edition drives. 8 meg cache on each, 7200 rpm... yummy. Run the 4 drives in a RAID 0+1 configuration (striping and mirroring) for speed and protection against drive failure. Or if you make regular backups, just get 2 drives and run them in RAID 0 (striping) for speed. Get a DVD+RW (4.7 gig rewritable, compatable with regular DVD drives, under $400) instead of the additional 2 hard drives and make regular backups.
I run 2 drives in RAID 0 and it's noticeably faster than a single drive.