Thanks for the reply's. I looked at the MB again and now see the 2 SATA connectors. I completely missed them before (I did look, honest).
So, I think I will buy 2 1TB SATA drives. Swap the parity first. Then add the other new 1TB drive (and, after that, add the old 500GB parity drive as a data drive). I'll probably also pick up a couple internal SATA cards. Some of my old drives are pretty small and will need to be replaced, when the time comes, with SATA drives.
Doesn't sound too bad. Now, if I can just track down that pesky problem with my slow network (only 10MB/sec throughput on a 1Gb switch in a full 1GB environment). Always something to "optimize"...