Just for clarification, PCI-X is a 64 bit bus variant of PCI that operates at 66-133MHz rather than 33MHz. And yes, PCI cards can be used on PCI-X interfaces and vice versa.
I assume however you are talking about PCI-Express which is written as PCI-E. PCI cards will not work in PCI-E slots and vice versa as you're looking at complete different fundamental designs
PCI-X is rare and general only found on server/workstation grade boards, and has pretty much been replaced as PCI-E for high bandwidth periphirals