Good question.
The 10K rpm Raptor drive is FAST. But it isn't cost effective. I currently use two older SCSI Ultra2 drives on a Dell Perc2 controller in a RAID0. It is very fast all around. Complete cost of the controller card and two 36GB 7200 RPM SCSI drives was less than one Raptor at the time.
Currently onboard RAID solutions from any of the major three (Intel, Nvidia, AMD) are pretty good compared to even a few years ago. For $100 plus dollars you can get one 36GB Raptor drive, or for about $90, two Hitachi 160GB SATA drives on
www.zipzoomfly.com and make something that is quicker and a little less expensive. Yes, RAID0 is more prone to data loss, but you can run a RAID 0+1 on the newer Intel ICHxR or Nvidia chipsets and get the best of both worlds. Redundancy and performance while also getting more storage and more HD throughput. I'd go RAID0+1 instead of the Raptor.
Of course you could RAID0 two Raptors or buy 15K rpm SCSI and make us all jealous!