You would have to look at the performance/price ratio of the 6850 vs 7850… I don't know. That is, with a 6850 you can play BF at high settings, with a 7850 at ultra. Always remember that two graphics cards mean double consumption, double noise and in the future it would be complicated to add a third graphics card.

PD. Uhh well the P67 Fatal1ty Professional is an excellent motherboard for high OC, the Z77s are more modern and allow the use of the integrated graphics and have some extra features for SSD PCIE 3.0 etc. but they are less dedicated to OC. Perhaps I would opt for the first one, but I think it's interesting to have PCIE 3.0...