I have a amd 1090t as my cpu, and I was just wondering if thats more so holding back my fps in MMO's or would the 660 make a decent jump for improvement?
It depends on the MMO, your graphical settings and your screen resolution:
The 1090t isn't as fast in games as any Sandy Bridge i5 or newer, but it's still a decent performer. The 560Ti will probably need some settings turned down in things like the new WoW expansion or GW2 at 1080p, and a 660 will probably give you a slightly smoother experience. It also depends on your expectations; The 1090t is never going to be as smooth as something like a 2500K - your minimum framerate will be lower and it will definitely stutter more. Maintaining a silky-smooth 60fps at v-sync is going to be difficult in a modern game with any AMD chip. On the other hand, the benchmarks do clearly show that something like a 1090t will still give you high average framerates with all the detail cranked up. If you are the sort of person who doesn't mind playing without vsync, and can tolerate a few more moments of stutter I see no reason for you to upgrade your CPU.
Personally, I wouldn't waste your money on the upgrade to a 660 - the measured FPS figures make it a 25% upgrade, the 99th-percentile 'perceived' upgrade means it'll feel
a bit better than that, but still not enough to justify the cost in my opinion. Save for a GTX670 or if money is tight, the HD7950 offers better performance/dollar assuming your PSU and case can handle one.