Source: www.wikipedia.com/wiki/pci_express
PCI-SIG announced the availability of the PCI Express Base 2.0 specification on 15 January 2007.[4] PCIe 2.0 doubles the bus standard's bandwidth from 2.5 Gbit/s to 5 Gbit/s, meaning a x32 connector can transfer data at up to 16 GB/s in each direction.
PCIe 2.0 is completely backwards compatible with PCIe v1.x. Graphic cards and motherboards designed for v2.0 will be able to work with v1.1 and v1.0, and vice versa.