I have two x16 slots. Basically for best performance in wow (vista), you want to hang both monitors off your GTX260, and use Dual View in the nvidia control panel. Well that seems to work best for me. Putting the second monitor on my 8800GT caused very bad FPS. Turning Aero off helped my performance as well.

I'm pretty sure, that games can enumerate adapters (video cards) and choose which adapter to render to. (Supreme Commander supports multiple monitors for instance.) I'm assuming from my reading that Wow only renders to the primary adapter, so that is why one video card is best (unless you do some hacks like I linked to in the post above).

Dual View lets you have the two monitors at different resolutions.