PCI Express 101
 

For those who are looking for an upgrade to their system, PCIe (short for
PCI Express) is a great technology to take advantage of. However, if you
are looking to have the best possible graphics, there is no reason to
splurge on a motherboard just with PCIe as AGP 4x and 8x should provide
ample bandwidth for any intensive games you run, including Doom III / Half
Life 2 and other soon-to-be released titles.

http://www.viperlair.com/articles/editorials/pcie/