Just sharing the summary from this very detailed review results of the GTX 660 Ti from X-bit.
Basically i would say it is positioned somewhere here: HD 7950 >
GTX 660 Ti > HD 7870
Compared to HD 7870QUOTE
They are identical in price but not in performance. The Radeon HD 7870 GHz Edition wins in DiRT Showdown, Sniper Elite V2 and (at the heavier settings) in Metro 2033: The Last Refuge, Aliens vs. Predator (2010), Sid Meier's Civilization V and Batman: Arkham City. But the rest of the games and benchmarks are won by the GeForce GTX 660 Ti. The latter is an average 12-17% faster at 1920x1080 and 6-13% faster at 2560x1440. That’s good but only if the GTX 660 Ti costs as much as the Radeon HD 7870 GHz Edition costs now.
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Compared to HD 7950QUOTE
The Radeon HD 7950 improves AMD's position even more in the games where the Radeon HD 7870 GHz Edition was superior and adds a win in Crysis 2 to that. The other tests are still won by the GeForce GTX 660 Ti, but by a smaller margin. Mathematically, the Radeon HD 7950 is faster than the GeForce GTX 660 Ti across all the tests (by -2 to +6% at 1920x1080 and by 5 to 14% at 2560x1440).
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Compared to GTX 670QUOTE
The gap is actually not very large. The GTX 660 Ti is an average 13-19% slower than the GTX 670 at 1920x1080 and 15-23% slower at 2560x1440. That’s not much considering that the GTX 670 costs 33% more. The effect of the narrow bus can be seen again, by the way: the gap is wider when antialiasing is turned on.
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