1680x1050 - High Quality
Testing at 1680x1050 with the very high quality preset and no anti-aliasing revealed that Far Cry 3 is still very demanding. For an average of 60fps or higher, you'll need at least a Radeon HD 7870/7950 (62fps). If you can get by with less than 60fps, the GTX 660 would be a fine pick with 55fps, a fraction slower than the GTX 580.
Speaking of previous generation flagship cards, the HD 6970 averaged 51fps, slightly faster than the 7850, which matched the 5870 and GTX 480 with 46fps. For around 40fps, you'll need either a GTX 560 Ti, GTX 650 Ti or HD 6870. Going below 40fps results in choppy gameplay, and this was certainly the case with the HD 7770's 32fps.