If you grab a couple of 7200rpm 16mb cache drives and stick them in RAID 0 you'll get excellent performance, and a lot of storage. The raptors are all about speed, not storage. The only thing better than 2x74gb raptor in RAID 0 is 2x150gb raptor in RAID 0, but that will run you a lot of money.
So for your dream machine, 2 150gb raptors in RAID 0. That's about $550, but its unbeatable in terms of speed (unless you consider SCSI but thats mostly a server technology)
For the medium rig, grab some 7200rpm 16mb cache drivers and you're good to go.