5GHz PCI-E clock....
(It also seems like overkill. Since when did you hit a PCI-E bottleneck...)
Not according to PCI-SIG ratification of PCIe 2.0 spec. 8)
(Although they don't say GHz but GT/s
DailyTech: PCIe 2.0 Ratified
PCI-SIG: PCI Express® Base 2.0 Specification
...The higher bandwidth will allow product designers to implement narrower interconnect links to achieve high performance while reducing cost.