First off..  no matter what vid card you have a G4 350 is never going to be able to play HD video.  You would need to upgrade it to a dual G4 1GHz+ to be able to do that.

About the card..  a 9800 is too much power and money for video plus it will only be feeding through a 2x AGP slot.  I have seen a few 64mb radeon 7000 agp flashed for mac on ebay.  that would be all the video power you need and they are only about 45-50 bucks.
Mac mini C2D 1.83GHz - 2GB RAM - 200GB 7200rpm HD
Sawtooth w/G4 1GHz - 1GB RAM - SIIG SATA w/2x1TB HD
B&W w/G4 600MHz - 1GB RAM - 2x80GB HD
Sawtooth G4 400MHz - 512MB RAM - 40GB HD
Dell Ultrasharp 2007WFP 20" Widescreen LCD
2.94TB Total Storage