2 GPUS in Vista may also cause a problem/Bug. Check out the sticky to the fix.

Possible fix for some of you with Vista and poor Graphic Card preformance low Fps

http://support.microsoft.com/kb/936710
" When a DirectX 10 application runs on a Windows Vista-based computer that has multiple graphics cards, the computer does not use the secondary graphics card"

  1. You have a Windows Vista-based computer.
  2. The computer uses two or more DirectX-compatible graphics processing units (GPUs) in a linked configuration.
  3. You are using a DirectX 10 application and driver.
In this scenario, the computer's graphics performance indicates that only a single GPU is being used.
CAUSE
This problem occurs because the operating system does not forward driver-render requests to the secondary GPU.

This may also be a problem. Anyway you can check it out. This problem will effect more than just running wow.