I assume from your post that your running duel monitors on this rig. There are several issues here, and a few things I need cleared up before I can really address your question properly, but I will try.

A key item here is your OS. From memory (and it’s been quite some time since I played with duel monitors from one rig) I think Windows XP had some overhead issues, especially when running two DirectX programs on different monitors. You will find that running those same two programs on the one monitor (being careful not to overlap any part of them) would run far better then on a duel configuration. I believe this was fixed, or at least fine tuned more in Vista (anyone able to confirm or correct?) I do doubt though that you’re running Vista on that rig, so likely this may be a large part of your problem.

Second thing that stands out to me is the lack of video memory. Though you don't mention your processor speed, at this point I would personally be looking at upgrading the video card to something with at least 512mb and (assuming your using the integrated 6150LE for one monitor, and the 7900gs for the other) disabling the integrated video all together. Not only is the 6150LE quite underpowered, but it's also sucking on your 2gb of system memory.

I don't believe your going to see this configuration ever run two WoW clients without slowdowns and if you can I would advise moving them onto one screen instead of two, but I feel that if you address the video cards first, especially from what you’ve said about the anomalies on one screen, then you will likely see some improvement (though I really cant say how much)

- Aldercy