MMOs aren't usually quite demanding but these days,the limiting factor in most games are GPU bound. Anything over a quad core will do about the same job, so I'd spend a bit more on the GPU. But it all depends on your budget
Its hard to say some MMO's are CPU heavy and some aren't... There are a bunch of people say that Wildstar runs poor on anything slower than a FX 8 or i5 no matter what GPU you have.
Like gaming in general the only solution is more power from both CPU and GPU. They are both equally important.
Something like a i5 4670K and a R9 280X would give you superb gaming experience and something like a FX 6300 and a GTX 750 Ti would give you a good gaming experience.
There's no easy answer especially when you consider optimisation and driver support. Performance isn't universal.
I'm running Wildstar @1080 max settings with my Phenom X4 965 and a 770 with zero issues, just like every other game I play. Anything with Wildstar at this point can be blamed on server issues I am sure, because they are having a shit ton of them.