Don Woligroski recently published Far Cry 3 Performance, Benchmarked, and determined that even the Low quality preset looks pretty darned good. In his performance evaluation, he ran around a specific outpost during daytime. Because there is no built-in benchmark for Far Cry 3, we end up with some variation in our results due to user input. At least we end up with numbers representative of actual game play, though. We're replicating Don's methodology for benchmarking AMD's mobile graphics processors.
At the Low quality preset, the 7670M is fairly playable at 1600x900, and definitely workable at 1280x720. Dips as low as 17 FPS at 1920x1080 render that setting untenable. The Radeon HD 8790M alleviates all of that, serving up anywhere between 50-60% more performance. Even at 1920x1080, the game is fairly fluid.
Stepping up to the High quality preset renders the Radeon HD 7670M completely unplayable. The 8790M gets by at 1280x720, but is hampered by demanding graphics at 1600x900 and 1920x1080.