after reading your text a several times, I had one more idea. At first, you gpu is not the best, but good enough for playing WOW
in Ultra. Second, there is no bottleneck using your gpu with your cpu.
This is really a nice combination.
Last but not least: Have you installed directX? If not, please unistall the graphic drivers, first install directX, than the graphic drivers and than erase the folders "WTF" and "Cache" in the folder WOW .
That should work (found it on a german WOW-Site a few weeks ago :-)