You haven't mentioned a controller for the SCSI drives. If you are planning on RAID, that will add considerable expense and, quite possibly saturate the PCI bus. If I remember correctly, the PC-DL does not have multiple peer PCI bus architecture and has a cap of 133MB/s on the PCI bus. Two 15K drives in RAID-0 could either come very close to or saturate the bus, especially when other devices are connected. Those drives are 80 pin SCA and will need adapters. You will also need cable and terminator. If you do go this route, I'd suggest the Fujitsu MAS or Maxtor 15K drives, in a 68 pin connector, for best performance.
Normally, I'm a SCSI addict, but since you've got onboard SATA RAID that does not share PCI bandwidth, why not save some headaches and get exceptional performance with some Raptors?