Its all meaningless. You can compare a fill rate, teraflops and all that crap but at the end of the day it comes down to which is actually faster in game.
Its not utterly absurd for a lower tier card to be able to OC and match a more expensive one. But a 680 overclocks too, thats not to mention that fact that a 660ti cant OC itself out of being struck down by its 192bit memory.
Try running Unigine Valley benchmark at 1080p maxed out. A 680 gets about 45FPS, that will give you an actual idea of how close you are.
I am not trying to beat a 680, I know the 660ti cant because of the 192-bit interface. I am just showing that you dont have to spend $500+ for a graphics card to get good performance. My 660ti runs all my games max settings 60+fps. Once games change I will SLI it, but for now I am happy. I also know that a 680 can overclock
But think about this your going to spend $200 more dollars for how much more performance?? My 660ti just closed the $200 gap