Definitions of PCI Express on the Web: (G00GLE

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* an emerging (2004/2005) standard for high-speed graphics, likely to result in a 20% boost over 2003-era AGP 8x performance. The standard, supported by ATI and other vendors, delivers better power management, bi-directional simultaneous I/O and 4GB/s bandwidth.
http://www.pcbuyersguide.com/PCbuyersguide-Glossary.htm * PCI Express (formerly known as 3GIO for 3rd Generation I/O, not to be mistaken with PCI-X) is an implementation of the PCI computer bus that uses existing PCI programming concepts and communications standards, but bases it on a much faster serial communications system. It is being supported primarily by Intel, who started working on the standard as the Arapahoe project after pulling out of the InfiniBand system.
http://www.en.wikipedia.org/wiki/PCI_ExpressIn short,PCI express is faster than AGP and it is backwards compatible with other devices.