If you aren't getting X with a default Debian Lenny install on a 400MHz Apple PowerBook G4 Titanium ATI Rage Mobility 128 M6 (got all that Google?)
Try this xorg.conf - the 'Modes' section and the UseModes line are the additions I think - I kept tweaking it until it stopped being broken so other things might not quite be standard...
# xorg.conf (X.Org X Window System server configuration file)
Section "InputDevice"
Identifier "Generic Keyboard"
Driver "kbd"
Option "XkbRules" "xorg"
Option "XkbModel" "macintosh"
Option "XkbLayout" "us"
EndSection
Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
EndSection
Section "Device"
Identifier "Configured Video Device"
Driver "radeon"
BusID "PCI:0:16:0"
EndSection
Section "Modes"
Identifier "Modes0"
Modeline "1152x768" 64.994 1152 1178 1314 1472 768 771 777 806 +HSync +VSync
EndSection
Section "Monitor"
Identifier "Configured Monitor"
UseModes "Modes0"
EndSection
Section "Screen"
Identifier "Default Screen"
Device "Configured Video Device"
Monitor "Configured Monitor"
EndSection