Use one pci-e slot will be x16. Using two pic-e slots this board will reduce both slots to x8. X8 pci-e slots run great and u will not notice any performance drops.
The Asus z68 is a great board. Overclocks great and very reliable. The only thing is the 3rd pci-e slot cannot be used for tri-sli due to it x4 limitations.
So it depends on the amount of graphics cards you have in, not which slot?