@Sylvanas yeah, yeah i know its old but just thought i should bring it up
And for the pci-express slot, yeah it has nothing to do with running crossfire/sli (should have separated that from the rest of my post) but the OP did ask about it being a potential bottleneck so i thought i should share my experience with oc'ing it.
I don't think the drivers for crossfire determine the chipset, since it works on practically every intel board (and nforce 4) with 2 pci-express slots (correct me if im wrong) and i am pretty sure (not 100%) the ati drivers don't look for the chipset. But the nvidia display drivers do definately look for the cards + chipset.