I found a solution by myself - I've changed the keylayout to "british". The benchmark results are:

PowerMac G5 ATI Radeon 9800 pro 256M Mac special Edition

Quake III Arena
GL Driver: Default
Gl Extensions: On
Video Mode: 1024x768
Color Depth: 32
Fullscreen: on
Lighting: Lightmap
Geometric Detail: High
Texture Detail: default (looks below middle)
Texture Quality: 16
Texture Filter: Bilinear

timedemo 18
demo four.dm_68

1260 frames 38.4 sec. 32.8 fps

PowerMac G5 ATI Radeon 9800 pro 256M Mac special Edition

Quake III Arena
GL Driver: Default
Gl Extensions: On
Video Mode: 1280x1024
Color Depth: 32
Fullscreen: on
Lighting: Lightmap
Geometric Detail: High
Texture Detail: high
Texture Quality: 32
Texture Filter: Trilinear

timedemo 18
demo four.dm_68

1260 frames 44.1 sec. 28.6 fps