you have to upgrade the chassis, too, sorry.
the only chassis that works with PCI-X Cards to a PCIe Mac Pro is the Magma PE6R4.
No other chassis will do what you need to do.
I am in the same boat as you are, thatīs why I know.
here are some links:
http://www.magma.com/comersus_viewIt...?idProduct=183
http://www.digidesign.com/index.cfm?...00&itemid=5031
the second link does not work any more because of the new AVID website; but I posted it because maybe somebody know how to find that site on the new avid website....
sorry for the bad news; but just out of curiousity: why do you want a Mac Pro? I am totally happy with my setup, no problems, stable as a rock, and it does everything I need....
Cheers!