Just a little heads up for those considering building a new system. AMD has quietly released a bunch of inexpensive (relatively) socket 939 Opteron chips. Since they're socket 939 there are lots of mobos to choose from and they use regular unbuffered (not registered) RAM. They are available in single and dual cores with 1mb of cache or 1mb X 2 of cache for the dual cores. Also reports have them being GREAT overclockers with folks right off the bat getting 50% OC on air alone.

http://forums.pimprig.com/showthread.php?p=446310