PCI-E was created in 2004.
Each lane provided 250 MB/s. 16 lanes, the maximum on a single PCI-E bridge (slot), equal 4 GB/s.
However, since 2007, it was revised and edition 2 came out and was called PCI-E 2.0.
The main difference was DOUBLING speed; each lane now provided 500 MB/s, so the total speed u get on a single PCI-E 2.0 bridge is 8 GB/s.
PCI-E 3.0 is coming out soon; again, bandwidth has been doubled to 1 GB/s per lane, for a total of 16 GB/s per PCI-E 3.0 bridge.