I would not recommend using a Radeon 9800XT with the original Mac BIOS or a reduced BIOS as the fan speed control is not properly enabled without extensions that are only available under OSX.
I have already experienced this problem as I had a 9800XT that functioned perfectly initially, but is now artifacting.

Please learn from my experience.

I would recommend using a Radeon 9800Pro Bios for these cards. The BIOS can be suitably modified to run the GPU and memory at 9800XT speeds and the cooling fan will run at 100%, protecting your GPU.

Jim Igou (Iggy)