Our benchmark was a lengthy drive around the island, with pauses to hop out and fight various enemies that spawned along the way. Far Cry 3 randomly generates encounters as you explore; so we repeated the test multiple times on each card to ensure our frame rates were representative of the game's performance.
Far Cry 3 tolerates a low frame rate exceptionally well; the GTX 660's 28 fps is absolutely playable. The 7950 is significantly faster than the GTX 660, but the GTX 670 is the fastest card at High Detail. The Radeon 7950 seems to suffer from more texture pop-in than the GeForce cards, but that could be a problem on the software end. High detail levels with AA, AF, and DX11 are no problem.
Given the way ambient occlusion is handled in game, we opted to test performance with that feature disabled via profile editing. Both the 7950 and the GTX 670 pick up a few frames here as a result.
If I wanted to use Ubisoft's online store, I'd have bought the game through Ubisoft. Steam is nothing but a launch tool, it literally can't detect if the game executable is running, unless UPlay is. Talk about annoying.