As cjl says, 15k seems to be the highest available disk speed (and those will be SCSI one). Be aware that 15k disks are designed for use in servers that have very good cooling (and normally reside in an airconditioned server room). They may not be suitable for use in a workstation without additional cooling.
If the disks are no longer needed I would suspect that they're going to be relatively small ones. When you add in the cost of buying a suitable RAID card you'd almost certainly be better off buying some (cheap as anything) modern SATA drives instead.