What resolution are you running at? The only resolutions that may get close to requiring multi-GPU are 2560x1600, or anything multi-monitor.
I'm guessing you're probably 1920x1080 or below in which case any high end GPU from the new generation will last you years just fine. You'll probably find that performance in MMOs these days is more likely to be limited by CPU anyway, unless you're running a decent CPU I wouldn't invest too heavily in an awesome GPU.
A good rule of thumb is never bother with multi-GPU when 1 GPU will suffice, I'd always recommend something like 1 high end card instead of 2 mid range cards.