yeah, upgrading and spending 2-300$ on a GPU for only 20% more performance isn't worth it (like if you we're already running dual 560ti you wouldn't upgrade to a 670 for 300$ as it's only some FPS more but it cost a lot more...) In your case, the upgrade worth it as the 660ti vs a single 560ti is about 2 times faster so 100% more performance, now it worth some cash...
The 670 is about 10 to 20% faster than the 660ti and it's a lot better if you run multiple monitors and/or 3D or high resolution monitors (more than 1080p) as the 660ti is memory bandwith limited vs the 670 and high resolution (more than 1080p) start to show the bandwith difference from the 670 as it's pulling ahead a lot in this case...
A dual 660ti setup will destroy a single 680 in single monitors 1080p resolution, however if gaming in surround with 3 monitors or 3D gaming, the 680 will destroy the 660ti...
Also the 670 is the best choice right now as it's 100$ less than the 680 and only 5-10% slower
I personnlly think that a 560ti is still powerfull enough to play all games (not maxed out) so it's more on your side, if you feel that you want more, need more, upgrade, if not wait to see the 700 series and the 8000 AMD series (it'll be out mid to late 2013)