AGP (Accellerated Graphics Port) runs a 4x or 8x system (uprader ti 8x praimerely later) but the PCI Express (Peripheral Component Interconnect Express) runs at a 16x system.The PCI Express was aproved in april 2002,but the first bracket with PCI technoligi was launched in 2004. the PCI-X is now replaysing AGP because of the higer speed.
Of all the signatures you have read,this is properly one of them