Don't know why JFar is having problems, but I have 2 Nvidia 280 GTX cards in my machine, both work Flawlessly and had them on Vista, and now Win7.
2 cards can increase performance anywhere from 0 - 200% - it depends on the game. Almost all modern games support SLI, and NVidia adds new profiles to the driver all the time to increase performance for 2 cards with games. General performance increase is 40-90% over 1 card from what i've seen. NVidia will give you similar numbers.
If you play extremely graphic intensive games (Crysis, etc.), 2 cards will benefit you immensly - if you play general things like Chess built into Win7, etc., it will not.
I suggest you post your system specs and the games you play.