Here's what I would do if I were in your situation. I would buy a FX-55 for futureproof purposes, that proccesor will be a really, really good proccesor for at least another year, and depending on what you plan on doing, can provide good use for another 2 or 3 or even 4 years down the road.
Next, get the nforce4 SLI board, but only get one of the 6800GT's, you probably won't find any lag or poor performace in anything with even one of these cards. Then, when a later time comes, you can add another 6800GT in SLI, or even two newer cards if you desire. The SLI board once again provides futureproofing.
Go with at least a 500 watt power supply.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w