Those people suggesting someone to buy old tech over new tech that can OC to higher performance than old tech should rethink their suggestions to the OP. OP, get the 7850 and OC. It's going to be a nice upgrade. If you can get a 7870, then do that instead. So to answer your question. Yes, it's very worth it. Overclock it, and it will be an even better investment. HD 7 series is made to OC.
I plan on buying the XFX HD 7850 Black Edition Double Dissipation, if they ever release the damn thing, so far newegg has the XFX HD 7850 Core edition and the Black Edition with 1 fan, OP go with the 7850 and then get a Ivy Bridge CPU for PCI-E 3.0.