There is a moderate difference between the 660 and the 670, however for that price I think you will be VERY happy with the card. it is only slightly less powerful than my Radeon 7870 and I paid $230 for it. It looks like the typical going price for a 660 is around $190 so you did quite well for yourself with that purchase.
GPU Benchmark Site
It will blow away anything you throw at it for the next few years I'm sure, including FFXIV :D
Edited, Apr 7th 2013 8:02am by DamienSScott