New CPU for Rendering

Go for the I7 with the z77 board, there would be a huge performance difference between that and your current system.

Well, if your budget fits the i7, then that is the best choice. The current FX-8xxx's give the i5's and sometimes the i7's a run in heavily threaded software (like rendering), but the i7 is definitively better.

However (pure speculation, and putting many "if's" into the equation) getting a FX-8xxx, or as pointed out a PII x6 (as long as it is with an AM3+ board) will save you some money over the i7 solution, and PD is looking to improve easily over BD by 15-20% at the same clocks, which would most likely put it over an i7 in highly threaded software.

Personally I would say wait for PD and see how it fares against the i7, but then one might say wait for Haswell.

The i7 will be a solid choice no doubt, betting on PD fighting against it might be a bit of a risk.


EDIT: Also want to point out your 750w power supply will be enough for any of the cpu's being mentioned.