There's a compatibility list
here, although I think that it's somewhat dated. If you want 3D support as well, don't go above a Radeon 9250. I have a Radeon 9000 pro in my A1, and it works well.
Seeing that you're talking about OS 3.9, I assume that you have a PowerPC A1200/A4000. Elbox recently released Radeon drivers for OS 3.x.
Here is their press release.
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