There are two versions of the X1900 for the Mac:
OEM Radeon X1900XT - For Mac Pro only, available only from Apple through Apple Store or as a BTO component when buying a Mac Pro.
ATI Radeon X1900 G5 Mac Edition - This card is based of the X1900GT released for the PC market. It is compatible only with the Quad G5 and other PCIe (dual core) G5 towers. This card is available retail through ATI or any of the more popular vendors like OWC.
Note: Neither of these cards is compatible with the AGP-based G5 towers...