Again, what will it be used for, and what software (OS, Apps, etc..) are you going to run? And what hardware?
The Dual Athlon MP setup is probable going to be the cheapest and still offer some good bang.
The best IMO right now would be a Dual Opteron system. If using it as a desktop then go with a VIA chipset. If using it for a Hard core workstation or server, then look at the AMD chipset boards made by Tyan.