|Apparently the Radeon X800 XT card is/was available as an AGP card or as a PCI-E X16 card.|
If it is/was a X800 XT PCI-E X16 card, it has a 6 pin PCI-E power socket and it probably came with this power wiring adapter:
You may already have the necessary 6 pin PCI-E connector available from your power supply wiring.
If not you will need to buy a wiring adapter:
8" 6-Pin PCI Express (PCIe) Power Adapter Cable - Turn Dual Molex Power into a PCIe Power Connector!
They are usually available locally from places that have lots of computer parts.
If it's an AGP card, it probably has a 4 pins in one row power connector - 4 obvious larger square pins - that you plug a standard floppy drive female molex power connector into. The wiring of most power supplies has two of those.
For a card with the X800 XT video chipset....
A 300-watt power supply or greater is recommended to ensure normal system operation.