whats my bottleneck with 2 instances of MoP?

I'm currently running a i7-920 with 6gb ram and 2 radeon 4830 xfired on 2x 1920x1080 monitors

just came back to wow, didn't have a problem in cata having wow on both screens.

currently I experience some fps drops in certain areas running one account, and with one account on each monitor the lag renders it unplayable.

trying to figure out what my bottleneck is, or if I just have to build a whole new comp.

thanks!