The 7850 and the 7870 are the same chip. The 7870 is higher binned, that is, it's guaranteed to run at "7870" speed. You're slightly more likely to get a higher overclock on a 7870 than a 7850, but it's all in the luck of the draw.
If you do intend to overclock, just make sure to get a card with a decent stock cooler if you're not adding an aftermarket solution. ASUS DCII is the best, gigabyte's windforce, MSI's twin frozor, and HIS IceQ are also good.