Have you considered the 6000 series. The 6850 or 6870 are great for Crossfire and the 6850 can be cheap. I think the main thing with the 7 series is the PCIE 3.0 which you mobo wont support. Also according to this chart http://www.videocardbenchmark.net/high_end_gpus.html
the geforce GTX 480 performs better then all of these. Ofcourse it all varies from game to game. Look at bench charts this site has some really great bench charts use them.