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Why would you compared an overclocked 5850 to a stock 7850 though? If you were willing to overclock the 5850, why wouldn't you overclock the 7850?
Also, you can easily find a good HD7870 for
$215 now (see coupon on the front page for $170 price range.
I am not sure why you keep noticing only the situations where a stock 7850 loses to the 5870 but ignore the situations where an overclocked 7850 would be much faster than a 5870 (Batman AC, Crysis 2, Civilization V, Anno 2070, Arma II, Dirt Showdown). I think if you mostly play games that run faster on NV cards, you should be looking at GTX660 then or at the very least be looking for a sale on a 7870 if you don't want to overclock the 7850 by 25-30%. Generally speaking HD7850 is not a great upgrade from a 5850 in the first place. I would say HD7950 @ 1100mhz is a good upgrade.