GTX 660 versus Radeon 7870 is a much better comparison than gTX 660 versus Radeon 7850. The 7850 can be found much cheaper than the 660 and the 7870, so IDK why you'd compare them. The 7870 is generally similarly priced (sometimes a little lower, sometimes a little higher, sometimes about the same) to the 660 and they have comparable performance on average, with neither card usually far behind the other when it loses.
The 660's advantages lie in PhysX support (if you play the very few games that properly support it such as Borderlands 2 and Batman:AC) and somewhat lower power consumption whereas the 7870's advantages lie in overclocking performance, OpenCL/Direct Compute feature performance such as advanced lighting features, and a few...