one card:
- good performance on main monitor; if another wow on 2nd then crappy on both

two cards:
- same as above; you can connect 4 monitors and have a wow client on each but performance is bad (not playable)

SLI:
- both cards drive 1 display only; wow does not use SLI and the actual pefromance will probably go down (that's what happend in my case)