In my opinion, given the stats that were released for the 8600, a single 8800GTX would out-do 2 8600 Ultras. Given more shaders and alot more memory. 2 SLi 8600s = 512mb (or 1gb, not sure yet, that hasnt been exactly clearified to my knowledge), 256-bit memory, 128 shaders. Single 8800GTX = 768mb 384-bit memory, 128 shaders. Well, as you see it looks to me close to an equal match. But your memory buss shouldnt change with SLi, so the 2 8600s would most likely have a much lower memory bandwidth, and that would slow them down.
To me the single 8800GTX would be a much wiser choice, as it would leave room for maybe someday buying another one and then having 2 8800GTXs. That and it will be a high end card for some time, the 8600s will eventually not be good enough to keep up. The single 8800GTX would be able to keep up with upcoming games much longer.
Also, the single 8800GTX would consume alot less power.
Im not sure how SLi exactly works, but im pretty sure it hasnt been completely worked out yet to be an effective gamer upgrade. Unless perhaps your using lower end cards like the famous 6600GT or Ultra SLi. I also am pretty sure that SLi can cause game instability as its not very reliable. From what i hear. I dont think i would trust SLi, i think its better to get the single best card then 2 of the lower end models.
