参考になるかわかりませんが、PowerBook G3のXorg.confを以下に載せておきます。PowerBook G3の場合も画面が乱れるので、仮想コンソールからpicoを起動し以下を入力して起動できました。
（以下　xorg.conf）
# /etc/X11/xorg.conf (xorg X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the /etc/X11/xorg.conf manual page.
# (Type "man /etc/X11/xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following commands:
#
#   cp /etc/X11/xorg.conf /etc/X11/xorg.conf.custom
#   sudo sh -c 'md5sum /etc/X11/xorg.conf >/var/lib/xfree86/xorg.conf.md5sum'
#   sudo dpkg-reconfigure xserver-xorg

Section "Files"
#    FontPath    "unix/:7100"            # local font server
    # if the local font server has problems, we can fall back on these
    FontPath    "/usr/lib/X11/fonts/misc"
    FontPath    "/usr/lib/X11/fonts/cyrillic"
    FontPath    "/usr/lib/X11/fonts/100dpi/:unscaled"
    FontPath    "/usr/lib/X11/fonts/75dpi/:unscaled"
    FontPath    "/usr/lib/X11/fonts/Type1"
    FontPath    "/usr/lib/X11/fonts/CID"
    FontPath    "/usr/lib/X11/fonts/100dpi"
    FontPath    "/usr/lib/X11/fonts/75dpi"
        # paths to defoma fonts
    FontPath    "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
    FontPath    "/var/lib/defoma/x-ttcidfont-conf.d/dirs/CID"
EndSection

Section "Module"
    Load    "dri"
    Load    "extmod"
    Load    "freetype"
    Load    "glx"
    Load    "record"
    Load    "speedo"
    Load    "type1"
    Load    "xie" # added from koos
EndSection

Section "InputDevice"
    Identifier    "Generic Keyboard"
    Driver        "keyboard"
    Option        "CoreKeyboard"
    Option        "XkbRules"    "xorg"
    Option        "XkbModel"    "macintosh"
    Option        "XkbLayout"    "us"
EndSection

Section "InputDevice"
    Identifier    "Configured Mouse"
    Driver        "mouse"
    Option        "CorePointer"
    Option        "Device"        "/dev/input/mice"
    Option        "Protocol"        "ImPS/2"
    Option        "Emulate3Buttons"    "true"
    Option        "ZAxisMapping"        "4 5"
EndSection

Section "Device"
    Identifier    "ATI Technologies, Inc. 3D Rage LT Pro"
    Driver        "ati"
    Option        "UseFBDev"        "true"
EndSection

Section "Monitor"
    Identifier    "Generic Monitor"
    Option        "DPMS"
    HorizSync    30-57
    VertRefresh    43-72
    DisplaySize    286 216 #PowerBook screen width height in mm
EndSection

Section "Screen"
    Identifier    "Default Screen"
    Device        "ATI Technologies, Inc. 3D Rage LT Pro"
    Monitor        "Generic Monitor"
    DefaultDepth    24
    SubSection "Display"
        Depth        1
        Modes        "1024x768" "800x600" "640x480"
    EndSubSection
    SubSection "Display"
        Depth        4
        Modes        "1024x768" "800x600" "640x480"
    EndSubSection
    SubSection "Display"
        Depth        8
        Modes        "1024x768" "800x600" "640x480"
    EndSubSection
    SubSection "Display"
        Depth        15
        Modes        "1024x768" "800x600" "640x480"
    EndSubSection
    SubSection "Display"
        Depth        16
        Modes        "1024x768" "800x600" "640x480"
    EndSubSection
    SubSection "Display"
        Depth        24
        Modes        "1024x768" "800x600" "640x480"
    EndSubSection
EndSection

Section "ServerLayout"
    Identifier    "Default Layout"
    Screen        "Default Screen"
    InputDevice    "Generic Keyboard"
    InputDevice    "Configured Mouse"
#    InputDevice    "Synaptics Touchpad"
EndSection

Section "DRI"
    Mode    0666
EndSection