Taken from
http://www.ccm.ece.vt.edu/~lscharf/samd/?topic=Linux&title=XFree86+on+Beige+Macintosh+G3 :

XFree86 on Beige Macintosh G3
Keywords:
Date Created: 2002-09-17
Author: Luke Scharf luke@vt.edu

The following is a sample /etc/X11/XF86Config-4 file that is compatible
with the Beige All-in-One Macintosh G3:

Section "ServerLayout"
Identifier "XFree86 Configured"
Screen 0 "Screen0" 0 0
InputDevice "Mouse0" "CorePointer"
InputDevice "Keyboard0" "CoreKeyboard"
EndSection

Section "Files"

RgbPath "/usr/X11R6/lib/X11/rgb"

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:unscaled"
FontPath "/usr/lib/X11/fonts/75dpi/:unscaled"
FontPath "/usr/lib/X11/fonts/100dpi/:unscaled"
FontPath "/usr/lib/X11/fonts/misc"
FontPath "/usr/lib/X11/fonts/cyrillic"
FontPath "/usr/lib/X11/fonts/Type1"
FontPath "/usr/lib/X11/fonts/Speedo"
FontPath "/usr/lib/X11/fonts/75dpi"
FontPath "/usr/lib/X11/fonts/100dpi"
FontPath "/usr/share/fonts/default/TrueType"
FontPath "/usr/share/fonts/default/Type1"
EndSection

Section "Module"
Load "GLcore"
Load "dbe"
Load "dri" # This currently kills kdm
Load "extmod"
Load "glx"
Load "pex5"
Load "record"
Load "xie"
Load "ddc"
Load "bitmap"
Load "freetype"
Load "speedo"
Load "type1"
Load "vbe"
Load "int10"
EndSection

Section "InputDevice"
Identifier "Keyboard0"
Driver "keyboard"
Option "Protocol" "Standard"
Option "AutoRepeat" "250 30"
Option "LeftAlt" "Meta"
Option "RightAlt" "Meta"
Option "ScrollLock" "Compose"
Option "RightCtl" "Control"
# XkbModel no longer used in XF 4.1.0
Option "XkbLayout" "us"
EndSection

Section "InputDevice"
Identifier "Mouse0"
Driver "mouse"
Option "ZAxisMapping" "4 5"
Option "Protocol" "IMPS/2"
Option "Device" "/dev/mouse"
EndSection

Section "Monitor"
Identifier "Monitor0"
VendorName "Unknown"
ModelName "Unknown"
option "dpms"
HorizSync 30-57
VertRefresh 43-72
Mode "640x480"
# D: 29.792 MHz, H: 34.482 kHz, V: 65.679 Hz
DotClock 29.793
HTimings 640 720 784 864
VTimings 480 483 486 525
Flags "-HSync" "-VSync" # Warning: XFree86 doesn't support accel
EndMode

Mode "800x600"
# D: 49.885 MHz, H: 47.967 kHz, V: 72.022 Hz
DotClock 49.886
HTimings 800 856 976 1040
VTimings 600 637 643 666
Flags "+HSync" "+VSync" # Warning: XFree86 doesn't support accel
EndMode

Mode "1024x768"
# D: 74.828 MHz, H: 56.346 kHz, V: 69.909 Hz
DotClock 74.829
HTimings 1024 1048 1184 1328
VTimings 768 771 777 806
Flags "-HSync" "-VSync" # Warning: XFree86 doesn't support accel
EndMode

Mode "1152x864"
# D: 79.911 MHz, H: 54.884 kHz, V: 59.917 Hz
DotClock 79.912
HTimings 1152 1216 1328 1456
VTimings 864 870 875 916
Flags "+HSync" "+VSync" # Warning: XFree86 doesn't support accel
EndMode

EndSection

Section "Device"
Identifier "Card0"
Driver "fbdev"
BusID "PCI:0:18:0"
EndSection

Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
DefaultDepth 15
SubSection "Display"
Depth 15
Modes "1024x768"
EndSubSection
EndSection

Notes:

1. This file has been developed and tested on Yellow Dog Linux 2.1. YMMV.
2. I used fbset to translate the modes in /etc/fb.modes into
XFree86-compatible configuration strings. Someone with more time might
tweak /etc/fb.modes so that the image is better centered on the screen
and fills the entire visible area - and then rebuild the "Monitor0"
section of this XF86Config-4 file.
3. In the "Modules" section, there's a little note about the DRI
module - the authors of YDL's XConfigurator claim DRI will cause
problems with KDM. I have not verified this, since I'm using GDM on my
test machine. If you have problems with KDM, please comment out that
line.