I'm not sure what you mean by PCI-X. Most boards have some combination of PCIe X16 slots, PCIe X8 slots, PCIe X4 slots, and PCIe X1 slots. Also found are some legacy (older) PCI slots. Any USB 3.0 adaptor card you find for any of those slots will work.
Thanks clutchc, I am dealing with an older PCI slot, and while I found a USB 3.0 card that will fit there, it can't offer any of speed advantage of USB 3 from PCIe.
You must be dealing with a server motherboard if you have a PCI-X slot. I'm not familiar with them. According to a quick research I did, they are a high speed version of PCI. http://en.wikipedia.org/wiki/PCI-X
I'll leave the server boards to someone wiser than myself in that catagory. Do you have a free PCI slot? Or a PCI express slot?