I don't know if you can fold using x-fire; might want to check the FAH forums for the answer. The correct client for GPU folding is the 5.91beta6. I hope you have proper cooling, since folding generates a lot of heat, 24/7.
With your setup, I would rather fold using your QX6700, Vmware and 2 Linux instances. That's what I'm using for both my Quads. Check this thread:
Linux SMP folding using XP / Vista + VMware
John