I use two 8600 GT Nvidia cards in my box and aside from it taking a bit of effort to get them configured they work fine. I am a relative noobie so it probably took me longer to set them up then it should have. Ignorance is not bliss in this case.
Perhaps if you posted a general hardware description of you system and what you use your computer for the more knowledgeable members of the list can give you a better answer.
If you don't like it, make something better
If you can't make something better, adapt
If you can't do either ball your panties up and cry
There was a new Sheriff in town but he got shot.