I think, good machine for testbed4 would be dualcpu opteron with nforce4 pro chipset. 2ghz should be enough.
Future is going to be dual core. But when you look at the first benchmarks, dualcore opteron has the same performance as the dual cpu opteron system.
So therefore I think dual cpu opteron system will be good choice for future.
Second, chipset. I think nforce4 pro will be good, because:
1. it has ata-II with ncq support.
2. many users in the world will buy nforce4 desktop chipset which has the same sata controller, so they can compare directly
3. nforce4 supports pci-e, i think in future, threre if there are going to be any new controllers, maybe SAS, threy're going to use pci-e
4. the board will have pci-x (or pci-64?), so you can use contemporary high performance controllers
Another suggestion is, to test contemporary new gen drives (wd300, 320jd, diamax10, hitachi 500gb, barracuda8, maybe new sata-II samsung and hitachi) on testbed 3, since testbed3 still provides valuable information about performance and is pretty up to date.
Then when the new testbed4 will be operational, you can test these drives again, and see how big will the difference be. I hope that with new testbed, you will update your benchmarks (and benchmark patterns) to more actual too.