PCI-X
Developed as a faster PCI bus, this bus revision (PCI Bus revision 2.0) was designed for workstation use, since before PCI-E people realized that the 133mb/s bandwidth PCI offered was pittfull for a RAID10 SCSI card or a dual Fiber optic Network controller. Its bus width is 64bits wide (compared to 32bits on PCI) and comes in 33,66,100 and 133mhz varieties, the Slots look like e-longated PCI slots and are about as large as an ISA slot. It offered the much needed increased bandwidth that network admins needed offering 266mb/s(33mhz) 533mb/s(66mhz), 800mb/s (100mhz) and 1.07GB/s (133mhz) bandwidth, with the advent of PCI-Express this PCI revision is being phased out in favor of newer PCI-E slots. Normal PCI cards that have the notch at the begining of the card slot can be used in PCI-X slots, some are even 66mhz compatible (like alot of PCI video cards) and thus can opterate better in PCI-X slots. Otherwise, normaly the use of a regular PCI card in a PCI-X slot forces the PCI-X bus to slow down to 33mhz (unless the PCI-X slot is on a seperate channel from the rest of the PCI-X slots, which almost always isnt the case) and all other slots will slow down to 33mhz as well.