Interesting link regarding PhysX and multiple cards:
http://www.slizone.com/object/sli_cuda_learn.html
I suppose my situation is the one they label "Multi Card". Seems it's possible to "Use the NVIDIA control panel to take complete control over your PhysX configuration. Use one card for graphics and dedicate a different card for PhysX processing for game-changing physical effects. Or, use 2 or 3 identical graphics cards to do both SLI and PhysX to experience maximum, heart-stopping realism."

As I understand it, WoW doesn't really benefit from PhysX. But I suppose if I have any other games that use it, it will be nice.