Well, since today my 9800XT 256M is arrived I did some tests and here they are!

PowerMac G4 ATI Radeon 9250 128M:
1260 frames 18.7 sec. 67.3 fps

PowerMAc G5 ATI Radeon 9650SE 256M:
1260 frames 30.9 sec. 40.7 fps

PowerMac G5 ATI Radeon 9800XT 256M:
1260 frames 50.5 sec. 24.9 fps

All three tests did with the same settings:

PixelFormat: Color(32 bits) Z (16 bits) Stencil (0 bits)
Mode: 6 1024x768 Fullscreen Hz:N/A
Rendering Primitives: Single Gl Drawer Elements
Texture Mode: GL_LINEAR_MIPMAP_NEAREST
Texture BIts: 32

used the same commands for all three, once I activated console:
timedemo 18
demo (name of the demo)

Then I played a couple of games and Quake III crashed (only the game, not system). Glitches and slowdowns in the first game I played.