The 7850 will give you way more performance and the Asrock Extreme3 Gen3 Z68 will give you 8x/8x if you plan to Crossfire/SLI in the future. The CX600 will be able to supply enough power for Crossfire 7850. The 7850 can overclock to the performance of a GTX 580 easy as many reviews point out. Also the power requirements for it are so little the CX600 will be able to handle 2x7850+i5 2500K overclocked.
With the current Z77 board you have, you won't get PCI 3.0 anyways since SB doesn't support it, you might as well get a cheaper board that still supports PCI 3.0 if you end up upgrading to Ivy Bridge but also supports 8x/8x.