Yeah 680 and 670 are very close, they both have the same 256bit memory bus, so its purely the additional cuda cores of the 680.
A 680 is easily overclocked to gtx 770 specs. 670s can get close too.
At stock speeds the 670 is often compared to the 760, but once overclocked the 670 is superior, especially if it has Hynix or Samsung vram.