according to this http://rog.asus.com/51612011/guides/nvidia-sli-and-amd-crossfirex-setup-guide/ you can pair any amd graphics cards together . where can i find reviews with different cards paired .
anandtech has 'bench' program that can compare components you want to see (that's listed there). And it also has Crossfire and SLI for each famous cards. http://www.anandtech.com/bench/GPU11/188