Test the PSU first without the 5850 and then with it. But I'd say it's very possible that the mobo didn't like the current draw of the Q6700...
I had the same system running on that Antec Earthwatts 380W PSU for over 2 months before change to SG06. I played COD:MW2, COD:Word At War, Battlefield: Bad Company 2, Metro 2033, Supreme Commander 2 and other games without any problems. When changing to SG06, only thing that changed was the PSU. 10 minutes of heavy gaming and mobo died. If current draw of Q6700 was too much I think it should have died during that 2 months.