To know for sure if your board will work in a PCI-X slot, you would need to know the board model and the revision. You can also tell by looking at the PCI connector to see if there is a slit in it. I think that this Knowledgebase will help to answer your question:
http://digital.ni.com/public.nsf/websearch/27BC5D80CDDD629486256BEB006A57F0?OpenDocument
Regards,
Erin