Try adjusting the FFXIV graphics settings first - some of the settings have a big effect on the frame rate. I have a GT 420 in my standard desktop, and after adjusting the settings it's just playable in FFXIV even though it was fine in AION. A GTX 580 or higher should give a proper gaming experience, but it needs a more powerful PSU than the one you've got. I've heard that AMD cards give the best price/performance ratio and use less power, but I haven't tried them.