Your specs are considerabley better than mine. While I haven't been to Dalaran yet, I move around TBC content comfortabley running 5 clients; 4 run on low settings while the primary is right (better) of center.

Consider addons. Vuhdo (sp?) brought my system to its knees yet I can run Carbonite on all 5. Wierd.

If you're boxing with ISBoxer, check your settings again after the patches. I had to reset mine... something to do with those virtual settings files. They all went to recommended settings for my machine running a single client.