Using a PCI-E 3.0 compatible CPU with this, I would say as of right now, a 690 would be the single biggest card you could put in there... I think the 16x PCI-E 3.0 has more than enough bandwidth to handle it. SLI is a different matter though.
That motherboard will run any pcie based card you care to install.
The strongest currently might be a GTX690 or 7990.
The pcie speed is largely irrelevant since even these top cards are not slowed down by the older pcie2.0 spec.