SCSI will be the the best solution, and the fasted. I would love to debate anyone on this. BUT, it's a pretty penny. In order for SCSI to work to the fullest, any contoler card would be best on a 64 bit PCI-X 133 host. You'll only find those in servers and high end work stations.
While you can get the SCSI card on a slower 32bit bus, and it does perfrom ahead of the sata drives with more reliablity, SCSI u320 will in theroy have to split its throughput on the bus to reach 320 and then your just pushing 160 up and down.
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...