PCIe 2.0 doubles the bus standard's bandwidth from 2.5 Gbit/s to 5 Gbit/s,
Meaning an x32 connector can transfer data at up to 16 GB/s in each direction.
PCIe 2.0 is completely backwards compatible with PCIe v1
Graphic cards and motherboards designed for v2.0 will be able to work with v1.1 and v1.0, and vice versa.
There aren't many video cards yet that makes use of the extra bandwidth of the V2 specification