7770 is worse than the 6850 wouldn't go that route.
I would try and not go Z77 right now with inflated prices. Z77 isn't as good as everyone says honestly. Nearly not benefit from the USB 3.0 controller to native. Gen3 vs Native PCI 3.0 or whatever has barely an improvement also so if you can get a Gen3 board and save go that way.
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Would probably work out. Gives you an i5 2400, 6870 which will be much better. Perhaps wait for the i5 2400 to be replaced by Ivy Bridge as the Ivy version is cheaper slightly.