The 9800 needs more power than the PCI bus can supply. It has a molex connector on the card (the same type of connector on the back of most PATA hard drives or optical drives). In any G4-based machine, you simply plug a free pigtail from the power supply into this connector.
In the G5, only the optical drive uses a molex connector (SATA hard drives used a special backplane connector that combines power and data). The 9800 comes with a Y-splitter for this connector and an extension cable. Unplug the power cord from the back of the optical drive, connect it to the splitter. Plug one end of the splitter back into the optical drive, the other into the 9800. Done.
-A.