The question has already been answered in this thread, but here is the exact numbers that should work for what you are wanting.
Section "Monitor"
Identifier "Monitor0"
HorizSync 30-80
VertRefresh 50-75
Option "DPMS"
EndSection
Section "Screen"
Identifier "Screen0"
Device "Card0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 16
EndSubSection
SubSection "Display"
Viewport 0 0
Depth 24
Modes "1024x768"
EndSubSection
EndSection