AMD Radeon cards are the better value currently, but Nvidia cards on average are suppose to run a tad bit better. You can't go wrong with a high end card from either maker honestly. You could have identical systems, one with a GTX 680 and the other with a 7970 and run tests until you were sick of them and never find a clear "winner". Nvidia does seem to have better driver
support, so that may be a factor. I've heard more people having issues with 2xAMD cards than 2xNvidia cards, mainly because of the driver. The GTX 670's are coming down in price and they are proven performers, but the 7870 GHz edition and the 7950's are cheaper or around the same price as the 670. I know nothing I've said so far help you decide, if anything it muddies the waters a little bit! Sorry about that lol.
Also, you are upgrading other parts as well? What exactly are your plans? That would probably affect which way you need to go in terms of graphics cards.