Example of an AGP Graphics Card (172-pin)
PCI or Peripheral Component Interconnect is the original interface standard before AGP. This method allows peripherals to be connected to the motherboard by using the computer bus. In addition to graphics cards, other connection cards can be added or removed using the PCI expansion slots.
Example of a PCI Card (120-pin)
Currently, PCI-Express or PCIe is the newest standard in graphics card technology and exceeds in performance beyond PCI or AGP. PCI-Express cards are available in four types: x1, x4, x8 and x16. They provide superior bandwidth capabilities allowing increased graphics processing with higher resolution and better 3D graphics rendering. PCIe is being adopted as the default form factor for a number of other hardware types such as network cards, wireless cards, modems, and sound cards. AGP and PCIe are not interchangeable. The index notch is in a different location on each slot.
Example of a PCI-Express Card (164-pin)
Diamond Multimedia. Glossary.
MCTSOL.com. Tech Center. What is PCI Express?