MMO games are more CPU dependent than a lot of games, but I'd say it depends on the game. A hyperthreaded i3 should be able to play any game pretty well though. Since you already have those parts, build your PC with them. Install but do not activate Windows (to save you from buying another copy if you decide to replace the mobo). Test it with one or more of your games and see how it does. How much improvement you need will determine your best course of action. The A4-5300 is pretty weak, but I can't predict how much improvement you'll want. The 750K would offer a decent amount, as will an i3, but an i5-3350P or FX-6300 would offer a lot more.