Don't get SLi...When you need to upgrade, if you get another 6800GS it will probably be outdated anyways, and you can't run SLi with 2 different cards. I suggest you get an EPoX 9NPA+ Ultra or a DFI Lanparty Ultra-D (you can mod the latter if you REALLY REALLY want SLi).
On the CPU side, skip the 3500+ and get an Opteron 144/146; price is similar and when paired with one of the above boards it OC's like crazy (FX55+ speeds not hard at all)
On the RAM size, just get some Corsair ValueRAM and save some $$. Not much performance difference, especially if you OC. Or if you get the DFI board, get RAM from another make such as Kingston or OCZ b/c DFI may have some compatibility issues with Corsair RAM.
Graphics card: Do yourself a favor and skip the 6800GS. Get a X850XT for less $$ (it's much faster and more OCable), or get a 7600GT if you want quietness and lower power consumption (it's about as fast as X850XT stock, which means it's faster than the 6800GT by a noticeable margin; 6800GT is a bit faster than GS, do the math)