If you are looking at a 5870 and a 5850 and have that much money to spend (about $700 total on average), you might as well go 5970 instead. It will be faster than 5850 in crossfire but slower than two 5870 in crossfire.
If you have a 5870 and are thinking of spending money on a 5850, I'd save up for another 5870.
If you have a 5850 and are thinking of spending money on a 5870, get the 5850 instead.
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