What is PCI-X?
PCI-X is the short for PCI-eXtended. It was developed in 1998. PCI-X is a computer bus and expansion card standard that enhances the 32-bit PCI Local Bus for higher bandwidth demanded by servers. It is a double-wide version of PCI, running at up to four times the clock speed, but is otherwise similar in electrical implementation and uses the same protocol.
PCI-X has been replaced in modern designs by the similar-sounding PCI Express, with a completely different connector and a very different logical design.
As you can appreciate in the below picture, the two long white slots are PCI-X while the small white slot is for a PCI.
PCI-X offered increased speed over PCI and has steadily increased to more than 30 times that of the original PCI bus. PCI-X clock increases the frequency of the data bus from 66 MHz to 133 MHz while conventional PCI supports up to 64 bits at 66 MHz.