Well I like to have very clean and fast drives for tracking/mixing.
So I have:
SATA-RAID drives for my OS/programs.
SCSI Ultra-320 RAID for tracking/mixing and a fixed swap file partition at the end of the drive. (inactive projects are archived then deleted from these drives so they are always clean)
External Firewire drive for MP3's, downloads, archives, etc.
But for most practical purposes, you can get away with only 1 drive. But I am a performance geek.