Express. Peripheral Component Interconnect Express to be complete. PCI-E (or PCIe) is the replacement of the old PCI, PCI-X (often confused with PCI-E and AGP bus interface on computer mainboards.
AGP is an older and different socket and there aren't many good cards for it these days. PCI-E is the newest, fastest socket and all the new cards support it.
PCI-X is an extension to the original PCI slot. It uses a 64-bit data bus (most but not all PCI slots are 32-bit) and runs much faster than PCI (PCI runs at 33 or 66 MHz, PCI-X ranges from 66 MHz to...