To answer your original question, even clocking the nuts off a 660Ti will not put it on par with a 670. Whereas many 670s will comfortably clock up to ~ 680 stock performance.
As to whether AMDs offerings are better on a bang per buck basis, there isn't a gulf of difference between them, so just research benchmarks on the games you play. If you find the 7950 offers comparable or better rates than the 670, go for it.
TBH though it's not that big a deal if you're gaming at 19x10 - both the 670 and the 79xx cards will monster most games at that res for quite some time*.
*uber hi res texture add-ons permitting.
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