Tiger didn't have the greatest support for aged ATI chipsets. Perhaps you should try a downgrade to Panther - that version essentially uses modified OS 9 drivers/resource files for the video (read the XPostFacto documentation for details), so graphics support is often better for older machines (which is the main reason why 10.2 runs on unsupported machines like the 2400/3400/Kanga, but 10.3 won't).

Otherwise, try either a Pismo or a PBG4, or a desktop box, for your graphics-intensive needs.