64 bit PCI and PCI-X are extensions of the PCI bus. According to Intel, the board you have can support up to 6 x 64 bit PCI cards at 66 MHz using risers.
So the first riser in my post above will work, but the second one won't work except for a single specific configuration. You'd need a universal keyed 32/64 bit PCI/PCI-X card that can tolerate 5 volts. The riser converts to 3.3v only keyed slots so plugging a 3.3v keyed card in will probably result in damage to the card. A universal keyed card would be fine though.
It's hard to find the correct riser because 64 bit PCI wasn't around for very long before being supplanted by PCI-X. They are supposed to be cross compatible if you have the correct voltage slots.
As for your Ebay link, yes that is an Apple part. The 820-xxxx-x gives it away. It may work since it is a purely passive adapter, but you'd be losing the extended functionality of the 64 bit PCI bus, both the bus width and speed.