I had the EXACT setup and I experienced a gain of anywhere from 20-50 frames on my games when I would disable crossfire. The chart that the guy posted is only an estimate, but sadly it is much more dramatic then you would hope. I was disappointed. If you want to crossfire/SLI, I recommend that all the PCI slots are x16. Experiment for yourself and you'll see. This is why on their website they have a list of compatible crossfire setups that give you good performance.http://www.asrock.com/mb/overview.asp?cat=VGA&c=CF&Model=P67%20Pro3%20SE
This pretty much shows that anything more intense than those graphics cards will be bottlenecked.