We currently have the exact same setup however I am on one computer as my girlie needed the other 2. I run 5 instances of wow but I am thinking of running 8 -10.

I would expect the video card to be an easy deal. I would not suggest SLI as I hear many good and bad things, there is no one answer on it. However you could just install another video card and not do the sli connection. Run them independently and you should be ok.

Let me know how this works out as I am thinking about the upgrades as well.

Oh 2 things I need to know

What video card are you using? I have the 4800 series.

Also have you tested to see how many wow clients will run smoothly on one cpu affinity. I am guessing that will determine whether or not 1 or 2 computers will do. If the cpu is choking at 2 instances then you have your answer. You need another computer. If it is barely noticable then I think you can just upgrade from there and everything should be ok.

My concern is not the video cards as the solution is easy but how the CPU handles multiple instances of WoW on each affinity.