that video card will work fine. as you can see in my sig, I'm running a similar card (Gigabyte 4850) that I've overclocked a little bit, and I run 5 instances pretty damn smoothly. Around 50FPS in my main screen, and 20FPS on my slaves. I'm sure I could bump my slaves max FPS up a little to 25 or 20 and see little to no difference performance wise, but I think I'll up some of the graphics settings on my main screen instead, right now I'm running it pretty nerfed out. I can run Crysis on Maxed out settings, but I haven't tried it with AA on yet, thats my next test of this card with crysis.

So, based on that, and the fact that the system you listed above is FAR superior to what I'm running, you should have no problem at all running many many instances of WoW.

P.S.
SLI motherboards are NOT compatible with ATI Crossfire modes. If you wanna run Crossfire, you need a crossfire ready Mobo, if you wanna run SLI, you need an SLI ready board, they are not interchangeable. Not currently anyways, and I highly doubt it will happen any time soon. But so far I'm extremely pleased with my card, I just upgraded it from runnin an SLI setup, mind you, the nVidia cards I was running were nothing compared to the new one (x2 XFX GeForce 8600GT's XxX) you can be sure that you'll be satisfied with the 4870, and if your considering crossfire, pick up a crossfire ready board, and the worst case scenario is you upgrade to crossfire somewhere down the road.

I would suggest that if you can afford it, pick up the 1GB version of your card. but thats a minor suggestion, that should be able to handle most of anything you can throw at it right now.