The compatible, qualified and certified Expansion chassis for ProTools and MacPro is the PE6R4-I model.
This is the best expansion chassis solution for Protools HD and MacPro ( Intel machine).
I own a Dual 2Ghz G5(PCI-X) not a Mac Pro(PCIe).
You can use either 4slot PCI expansion unit or a 7slot PCI expanion unit. Both expansion units are 32bit, they are not 64bit.