on par with stock GTX 660. clock for clock HD 7850 and GTX 660 are very close and trade blows depending on game. few games like Crysis 3, Farcry 3 the GTX 660 would edge ahead by 5%. few games like tombraider, hitman absolution,sleeping dogs HD 7850 would edge ahead by 5%.
here you can see a HD 7850(1100 Mhz) perf comparison with stock GTX 660.
If you're looking for good price/performance ratio, I recommend adding a second 7850 for crossfire. The 7000 series is getting a bit older now so it may not really be worth it anymore, but I'm still on par with a stock single 7950/7970 setups when overclocked.
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