Mmo's are primarily CPU intensive games, so a stronger CPU is always a bonus. Since you are running a pretty much balanced CPU/gpu, you could go either first, depending on what your preference in direction is. While decent speeds, the A8 is not a particularly strong CPU, as half the die is dedicated to the integrated gpu, sacrificing the 'L' caches. For an FM2, the best CPUs are the x4 750k or 760k, both are strong performers. These will help considerably in MMO's.
As for other game types like watchdogs and bf3/4,these are primarily gpu intensive, so an upgrade there to a gtx 760/770 or r9 280x/290 would be a huge improvement in fps over the gtx 650 you currently have.
As far as AMD vrs nvidia goes, that's all a matter of preference not performance,, as pretty much both trade blows, a 760 will trade blows with a 280x depending on the settings, the game etc. Es h has its strong points and weaknesses, nvidia having shadowplay, physX, etc AMD has eyefinity and generally better performance at high resolutions like 3x monitors. So, preference.
Only one thing to remember with a new gpu. Get rid of all traces if old drivers/software before installing new ones, or you will end up with driver errors from incompatible versions. This is especially important if switching from AMD to nvidia or vice versa. Either way, the CPU type/brand is unimportant to the gpu, it only cares whatslot it sits in.
Also, upgrade CPU, and there are no other upgrade requirements. Upgrade gpu and chances are good you will also have to upgrade the psu to matchthe power requirements and connections of the gpu.