First of all, welcome to the forums
Now, if you ask me, I would never SLI (nvidia) or Crossfire (ati) two video cards together. Thats because it is very expensive (double the price of the video card) And will most likely NOT give you a 100% increase in performance. Although say you have a 2x8800GTX setting, it will definitely run everything maxed out (except Crysis

) at fluent FPS that of course if you have a nice CPU and RAM.
But the disclaimer is this: you have your expensive top of the line $800+ SLI/Crossfire Setting and then what? a better line of video cards come out and you're outdated and will most likely want to get 2 of those new cards.
Overall, just pick one, the GTX is more than enough for today gaming and it will be easier for you to upgrade if new cards come out later. Also consider the 8800GT
Hope that helped
Haha pretty lenghty