$800's overkill, IMO.
If you don't mind a little OCing, get a 3200+ and either an Epox EP-9NDA3+ or a Chaintech VNF4/Ultra, depending on whether you need AGP or one PCIe x16 slot. For SLI, there's the Asus A8N-SLI out right now, but you're still CPU limited with 2 6800 GTs/Ultras even with an FX-55.