You can run as many monitors from a single PC as video outputs you have, however, the secondary video cards don't like to do 3D rendering very much. So as long as your are only intending to run WoW off of the main video card you should be set.

Honestly, with the setup you're looking at you'd be perfect. All instances of WoW running off of your "main" videocard, and anything else (non 3D) on the secondary video card. That is really all you need.