PCI-X changes the protocol slightly and increases the data rate to 133MHz (peak transfer rate of 1066 MB/s)
PCI-X 2.0 specifies a 266MHz rate (peak transfer rate of 2133 MB/s) and also 533MHz rate, expands the configuration space to 4096 bytes, adds a 16-bit bus variant and allows for 1.5 volt signalling
Also, i saw a PCI-Express 4x Slot on a mobo.