According to the settings we chose on a per-game basis, picked to maximize visual quality at playable frame rates, the GeForce GTX 660 Ti is close to, if not slightly slower than a Radeon HD 7870. We already know this runs counter to Nvidia's expectations, which put the new card between AMD's Radeon HD 7870 and 7950. However, after comparing lab results, the outcome of our testing appears tied to the way we picked settings for each game, likely taxing the 660 Ti's memory bandwidth more than less-demanding options would.
Perhaps our settings favour the Radeon cards. Perhaps theirs favour the GeForce-based boards. And maybe the most real-world outcome lies somewhere in between. But, I believe the truth is that GeForce GTX 660 Ti performs within 5%, plus or minus, of the Radeon HD 7870.