Well there are a lot of benchmarks on the web. Most of the GTX660 reviews show comparison to both the 7850 and 7870 as the card is priced right there between the two.
I was just trying to decide on a GTX660 and with the prices of the 7850 dropping like they have, I felt the 7850 was a better bargain. GTX660 performs a little bit better in a few titles, but not all and it's abut $40-50 more right now taking into account special deals.
For $250, though, I think the 7870 is the card to get.
gtx660 is more powerful than hd7850 but not by much at all.. in terms of price /performance gtx660 loses to hd7850 easily . if you havr a budget of $250 i recommend hd7870 which is a bit better than 660
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 others such as bit coin mining. I'd get the 7870, but the 660 isn't a bad choice at all.
Looking at cards based on 28nm construction, namely GTX6xx and amd 7xxx is a good idea. They run cooler and take less power.
The market for graphics cards in the $250 price class is very competitive.
You will get what you pay for at any price point.
The difference in performance between two similarly priced cards may show up in benchmarks, but will not be noticeable in actual gaming. If you have just one game you want to optimize for, then look for benchmarks using that game.
If you have a preference for AMD or Nvidia, go with that.
Otherwise, do not anguish over minor differences.
Just buy the best card your budget and PSU will allow.
The graphics card is the real engine of gaming.