Here's the thing you have to consider w/ your setup: yes, Vista is slower, but by how much? Depends on the game of course, but I haven't seen it slower by any more than 20% in a single game. So the question is what are the FPS you're getting now? Are they over 100? If so, then Vista is only going to be 20FPS slower which puts you at 80fps, well above the threshold of the human eye.
My point is this: yes Vista is slower, but when you've got a 8800GTS it doesn't matter because your games are blazing fast anyways.
I've got a 8800GTX and Vista... I have not seen ANY performance degregation in ANY of my games compared to XP.
The real question you should be asking is in regards to driver support. Do ALL your devices have Vista drivers? If so, I recommend upgrading. Vista is better.