I've had 10k rpm drives for a while, then upgraded about 2.5 years ago to a Maxtor 15k drive. If I could right now, I'd purchase an Adaptec SAS pci-e card (only RAID cards available ~$800) :mrgreen:. Still, I'd advice to go SCSI. Some of the Adaptec PCI-X 64-bit cards work in regular 32-bit slots, as long as you don't have any obstructions on the motherboard.
From my experience, go with a SCSI 15k for applications that need fast access like the OS, and have a 10k or 7200 for backup. I've had only Maxtors, but I wouldn't go with them since their software SCSIMax software does not support Ultra320 drives (they don't even mention SAS support). Wait a bit for the new Seagate 15k.5 drives...seek times not as fast as the Maxtor 15k IIs, but its worth it (extra space, and support).
Like the fellah before me said, 64-bit PCI-X slots only appear on workstation (ex.: Asus K8N-LR) and server boards (Tyan K8WE). If I had the dough, I'd buy them...