I have one latop whose monitor resolution is 1400x10050. And I have also use an external Dell Mointor whose resolution is 1920x1200.
Section "Screen"
Identifier "Configured Screen Device"
Device "Configured Video Device"
DefaultDepth 24
SubSection "Display"
Modes "1920x1200" "1400x1050"
EndSubSection
EndSection
Section "Device"
Identifier "Configured Video Device"
EndSection