Benchmarks: The Witcher 2, The Elder Scrolls V
Playing The Witcher 2 at 1920x1200 produced an average of 51fps when using the GTX 660, placing it 6% behind the HD 7850 and 19% behind the HD 7870. For whatever it's worth, Gigabyte's overclocked card managed to close the gap and meet the HD 7850 at 54fps.
The GTX 660 performed well in Skyrim, rendering 65fps at 1920x1200 versus the HD 7850's 54fps and the HD 7870's 63fps. It also outpaced the GTX 560 Ti by 51%.