PCI-X Raid card wont fit in motherboards PCI Express x16 slot????
At the risk of stating the obvious, you need a motherboard with a PCI-X slot to install and run a PCI-X card at maximum efficiency. In your case, you need a motherboard with a 133MHz PCI-X slot.
Some PCI-X cards can be run in standard PCI slots with a corresponding decrease in perfomance.
PCI-X and PCIe are completely different beasts (i.e, parallel vs serial). Trying to use a PCI-X card in a PCIe slot is like trying to connect a PATA hard drive to a SATA cable.