Here's a table with the detailed test results:
First of all, let’s check out the difference between the GeForce GTX 660 (priced at $229) and the GeForce GTX 660 Ti ($299 or 30.6% more expensive), the latter serving as a baseline.
The GeForce GTX 660 is 13 to 16% slower than its Ti cousin at 1920x1080 and 13 to 17% slower at 2560x1440 across all the tests. The difference in speed doesn't seem large considering the 30% difference in price.
Meanwhile, overclocking the GeForce GTX 660 to 1083/6668 MHz doesn’t always help to reach the performance level of the GeForce GTX 660 Ti as is indicated by the next pair of summary diagrams:
With just a couple of exceptions, the GeForce GTX 660 Ti is still in the lead. Other factors being the same, the lack of 28.6% shader processors in the GPU affects the performance of the GeForce GTX 660 so much that the higher clock rates can't make up for that.
Now the key diagram of this test session is GeForce GTX 660 2GB vs. Radeon HD 7870 GHz Edition 2GB at their default clock rates. The AMD-based card being older, it serves as a baseline.
Thus, the new GeForce GTX 660 is ahead in both versions of 3DMark, in Lost Planet 2, in the antialiasing mode of Just Cause 2 and in the MSAA-less mode of Batman: Arkham City. The two cards are equals in such games as Crysis 2, Hard Reset, Battlefield 3 and (at certain settings) in Aliens vs. Predator (2010) and Sid Meier’s Civilization V. The Radeon HD 7870 GHz Edition, in its turn, is superior in the high-quality mode of Unigine Heaven and in a lot of games: S.T.A.L.K.E.R.: Call of Pripyat, Metro 2033: The Last Refuge, Nexuiz, DiRT Showdown, Sniper Elite V2 and Sleeping Dogs. Its advantage is so huge in the last three games that it is an average 3-8% ahead of the GeForce GTX 660 at 1920x1080 and 7 to 9% ahead at 2560x1400 across all the tests.