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The Sauce
your 5850 *if* heavly overclock is already running faster than a stock 5870 ~ 480.
Basically your already above a stock 570, sure you could buy a 570 and overclock that, and probably gain like 15% or so faster performance, but is it worth $$$ to do that?
Why not give it a month, wait out christmas and buy a 7xxx series card, that could be ALOT faster, instead of giveing in now for virtually no gain, for a steep price tag.
The sauce:
Silly: to spend 200-300$ on a 570, only to get like ~15-20% more performance.
Smart: to spend 150-200$ on a used 5850, to crossfire for ~85% more performance.
Smartest: to wait for next gen cards, so you can buy 1 card, not have CF issues, and get real upg.