I can't speak to FFXIV in particular, but with Guild Wars 2 the AMD FX-6100 was a huge bottleneck, so much so that between a 550ti and and a GTX 660 I didn't see a noticeable performance jump. MMOs tend to be more CPU intensive than other genres, and the single-thread performance on the fx-6100 isn't too hot.
Depending on your budget, a jump to one of the nicer current gen i3 chips along with a GTX 750ti would likely be a pretty solid upgrade for around $300 total, assuming you get a motherboard/cpu bundle. This would leave you with the freedom to move to a faster i5 down the road if you were feeling like you wanted to get more performance, as at that point you'd already be on an intel board.
XI Wiki



