PCI-X is usually (if not always) backwards-compatible with PCI. However, unless you have a PCI-X (server) motherboard, they're not going to do you any good -- they'll run at PCI speed. Moreover, PCI-X implementation speeds vary; just because a (rare, server) motherboard has PCI-X support doesn't mean that it has PCI-X 133/64 support. Finally, PCI-X is fading, and if you want to do this with a forward viewpoint, and especially from a consumer hardware viewpoint, you might want to consider PCIe controllers.
On the server side, standards tend to last longer, and that's probably why PCI-X is popular in part -- because of its PCI backwards-compatibility. They'll be around for some time, but the newer hotness is going to be PCIe...