PCI-E or PCI-Express is the smallish version whereas PCI-X is the larger and older cousin. Most definitely not physically compatible. Recently I was asked to install a tape drive and its SCSI card into a server. The server specs list both PCI-e and PCI-X on the front page. Digging into detailed specs shows PCI-X is an option.
PCI-X is a parallel implementation of the PCI standard, slowly being phased out in favour of the serial version of PCI. That is PCI-Express.
Stuff like this can trip you up.