PCIe (PCI Express) is very different from PCI-X

PCI Express is the new point to point interconnect for desktop systems, primarily used by graphics card. PCIe can be bound together in multiple lanes, arranged in exponents of two. Current PCI Express chipsets support x1, x4, x8 and x16 modes, exposed as x1 and physical x16 slots.

PCI-X is an extension to the PCI standard for servers that has been around for a while.

You can find more specific information on PCI Express here.