PCI-X (Peripheral Component Interconnect Extended)
An extension to the PCI interface developed by IBM, HP, and Compaq. The extension allows data to be transferred at 1 GB per second using a 64-bit bus running at 133MHz. PCI-X peripherals are fully compatible with PCI and vice-versa, but will not run faster than the chipset or peripheral allows.
I cant believe how quickly I found that. There is never a decided term on this, I hate it. I've heard PCI-e called PCX for short. It's one of those things that is easily misunderstood.
PCI-E and PCX are not the same, as Nubius stated.
Basically, PCI-X is PCI on steroids, whereas PCI-E is a new architecture altogether.