No, they will not need to be the same, unless you want to sli/crossfire and then I'm not sure whats compatible with what.
Be warned that WoW always renders on the primary graphics card and you will get a performance drop when you try to move a WoW window onto a second/third graphics card as data needs to be pushed from the primary graphics card to the others.
I believe InnerSpace can make WoW render on the correct graphics card each time, but you might want to check that with Lax.
Here you go: http://www.dual-boxing.com/showthread.php?t=21233
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