I had similair fps issues with vista x64. The way I fixed it is disabling the "Desktop Window Manager Session Manager". It is the part of vista that does the "cool" 3d window tabbing thing and the preview windows on the start bar when you highlight running apps.

So for every open wow you have it renders it at least one extra time. I was 5 boxing and my fps was abysmal until I turned it off.

If you don't want to disable it permanently you can just stop the service when you play wow and restart it after.

Hope that helps.