This probably should've been written up a long time ago, but it wasn't. There's enough confusion about different standards. Let's start. PCI This baby is getting old and weary. A PCI bus is a 32-bit, 33MHz slot, which has a total bandwidth of 133MB/s. Due to today's high-speed devices, it doesn't take a whole lot to saturate this bus. Not only do cards plugging into the PCI bus use this, but there may be devices such as storage controllers or network adapters that use this bus. PCI-X PCI eXtended. This is PCI's bigger brother. Commonly found on server- and workstation-class motherboards, PCI-X is a 64-bit, 66, 100, or 133MHz slot, that delivers 533MB/s bandwith---if not more. It's higher bandwith is needed for SCSI RAID cards and Gigabit network adapters. Like PCI, it's a bus where multiple cards can be plugged into and where some integrated functions may be plugged in. AGP This is a 32-bit, 66MHz bus delivering 266MB/s. That's at 1x. Multiply the 66MHz by 2,4, or 8 and you'll get higher numbers in accordance with AGP transfer speeds. Good for up to 2.1GB/s. This is designed with video cards in mind, since the PCI bus isn't able to really push things through. I suppose PCI-X probably could've been implemented, but the slots are much longer than PCI or AGP slots, along with a higher pin-count---and that makes it more expensive to produce. PCIe/PCI-E PCIe x1 PCIe x4 PCIe x16 PCI Express, and often confused with PCI-X. Outside of the PCI part, PCI Express is vastly different from PCI, PCI-X and AGP. PCIe is a serial, point-to-point technology. This means, unlike a bus technology like PCI, PCI-X, and AGP, it doesn't have to share it's bandwith. PCIe slots are made up of lanes, each sporting 250MB/s in both directions (for a total of 500MB/s). Each slot is denoted by how many lanes it has going into it: 1x, 4x, 16x. PCIe is the universal replacement for PCI, PCI-X and AGP. Voltages and MHz remain the same between the slots as well, so any PCIe card can go in any PCIe slot. A few companies know this, and have arranged to have the back of 4x PCIe slots cut out to subtly encourage multiple video card use on non-SLI systems.