I guess that depends on whether they can flood an u320 bus. If they are 10k/15k rpm drives, I'd think so...
10k rpm drives. I can also get my hands on some retired IBM U320's if I want to pull them from our shop's boneyard.
No experience with that particular unit but Supermicro has a good name. That would leave whether your mainboard's bus/slot can handle u320 speeds or not if you do get a u320 card.
The X8SAX board I'm upgrading to uses a PCIe to PCI-X bridge chip to handle the two PCI-X slots. Based on my read of the manual, the x4 link used on the PCIe side of the bridge is sufficient to drive a pair of PCI-X 133 slots at full capacity.