two of the server’s slots accept pci cards up to 12 inches long; the third slot accepts a 7-inch card. (some models come with a card installed in one or two slots.) install only expansion cards that are compatible with mac os x and that comply with the pci 2.1 standard.
pci card requirements
criteria for pci cards you can install in the xserve server are summarized in the table below.

power consumption of cards
maximum power consumption for all three expansion slots combined should not exceed 50 watts (w). check the documentation that came with each card to be certain that the cards don’t exceed this power consumption.
important the pci card must have a 3.3 v connector or a universal connector to fit into the server’s expansion slots. cards with a 5 v connector will not work in this server.