I don't know what your budget is like, but I don't think the upgrade from an OCed 560 to a 7850 is worth it, especially not with your 2GB card. This depends a little on what games you play; compare here: http://www.anandtech.com/bench/Product/549?vs=543
For example, if you play mostly BF3, I definitely wouldn't upgrade.
It is true that the 7850 is a great performer for its price at the moment, but to make an upgrade worthwhile, I think you should probably aim a little higher if you can: the twin frozr 7950 that newegg has for $310 post-rebate will destroy your 560 and is pretty future-proof.
If you want an interim upgrade that will net you big performance gains, I'd consider getting another 2GB 560 and SLIing them (if your mobo supports it). That will yield nice performance at a lower price.
Finally, if you are committed to the 7850, I agree that the twin frozr is the best choice, based primarily on price.