SCSI (sky-see) drives require a driver card if you are talking about using them in a normal desktop, as they are not native to standard computers. They are (or used to be) very expensive for the capacity when compared to standard hard drives. Of course, a Raptor is not a standard hard drive, though.
If you could get a scsi drive or two and the driver card cheap enough to make it feasible, it would definitely scream. to me, though, I would put a couple of Seagate 7200.11 drives in a RAID0. Killer speed (almost a Raptor), more capacity, and cheaper to boot. But if your current mobo doesn't support RAID and/or SATA, you would have to get a card for it as well.