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
If you can't make it work maybe you are making it fail.