what frame rates are you getting? DO you have the frame rate locked? It might not be a problem.
Thanks for the reply,bud.
I didnt have my frame rate locked.When crossfired, on ultra setting i get like 50-60fps , worse in 64 player server(35-55fps) which is almost the same performance as my single card (even a little worse). However in single player, the frame rate is 70-110 on ultra while my single card get 45-60.