What is the Difference between PCI, PCI-X and PCI-e?
PCI-X is the upgraded version of PCI. It differs mainly in the fact that the PCI-X bus is 64-bits wide, and runs at higher frequencies of up to 533MHz, compared to PCI frequency that runs up to 66MHz.
PCI-Express, on the other hand, uses a serial interconnect along a switched bus dedicated exclusively to that slot. PCI-Express has the unique capability of multiplying up individual data lanes, to produce aggregate interconnects that can deliver up to 16 times the bandwidth of a single lane.
PCI-X is backward-compatible with PCI but not vice versa. The PCI-Express slot, on the other hand is no compatibility to either PCI or PCI-X.
As you can see in the below picture, PCI-X is an extension of the 32-bit PCI slot. The voltage is the same (3.3V or 5V), a PCI-card can be inserted into a PCI-X slot, although it will of course only operate at a bandwidth of 32-bits, and at the PCI frequency.