Lost our replacement PCI-X card today and want to replenish by purchasing another-
Would the listed will PCI-Express card MB842G/A fit in G5 2006 XServe RackMac3,1?
No. PCI-X and PCIe (express) are completely different interfaces. You can't plug a PCIe card into a PCI-X slot.
If Apple no longer sell the PCI-X cards you can find third-party options, or look in the re-sale market such as eBay. I may even have some PCI-X cards if you can't find them elsewhere.
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