Forgot to mention that I'm running main instance at 2560x1600 res with 4xAA, and this resolution is killing my card with shadows and other fancy stuff on even when singleboxing. Of course they are turned off when I 5-box.
I'm usually having 60-80fps, but when I'm in crowded area in dalaran or in wintergrasp or looking around that big open area with snowy trees (forgot the name) my fps can drop to 10-20.
And please stop speculating about other possible bottlenecks. I'm IT pro and I know for sure what causes my problems, I just dont want to type the long list of tests that lead me to this conclusion. I just want to know if adding another GPU would help, that's all.