PCI Express is also known as PCIe and is a bit-serial standard. In contrast, PCI-X (loosely, "PCI eXtended", but that's not an official name) is a highly parallel standard that directly extends the original plain PCI from 32-bit/33MHz to 64-bit/133MHz and faster. PCIe and PCI-X are entirely separate and different standards, and are not in any way compatible at the hardware level.
The basic PCIe slot employs one 2.5GHz serial signal path in each in/out direction, and that's referred to as "X1" or single-lane. To shift more data per second you can add more lanes, to give PCIe X2, X4, X8, X16, and higher. The current new generation of PCIe graphics cards uses PCIe X16 exclusively. They do not use any kind of PCI-X.
'Hope that helps.
