PCI, PCI Express, PCI X slots

PCI is the old bus system, coming in 32 bit and 64 bit variants. PCI X (Xtended) has compatible slots and runs at faster bus speeds. PCIe (Express) has completely different slots and fastest bus speeds.

For my reference, here are pictures of the different PCI slots courtesy of  http://www.adexelec.com/

Here are the different bus speeds, courtesy of OrbitMicro, which also sells expander/riser cards for these PCIe

Bus	Max Speed
PCI 33 Mhz 32 bit	132 MBps
PCI 66 Mhz 32 bit	264 MBps
PCI 66 Mhz 64 bit	528 MBps
PCI-X 133 Mhz 64 bit	1 GBps
AGP x1	264 MB/s
AGP x2	528 MB/s
AGP x4	1056 MB/s
AGP x8	2112 MB/s
PCI Express x1	500 MB/s
PCI Express x2	1000 MB/s
PCI Express x4	2000 MB/s
PCI Express x8	4000 MB/s
PCI Express x12	6000 MB/s
PCI Express x16	8000 MB/s