I assume you are sure that your monitor has a higher resolution.

In my xorg.conf file this is what I have:

Section "Monitor"
Identifier "Envision EFT920"
Option "DPMS"
HorizSync 30-100
VertRefresh 50-160
EndSection

Section "Screen"
Identifier "Default Screen"
Device "Intel Corporation 82845G/GL [Brookdale-G]/GE Chipset
Integrated Graphics Device"
Monitor "Envision EFT920"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1600x1200" "1280x1024" "1280x960" "1024x768" "800x600" "640x480"
EndSubSection
EndSection

If you dont have something similar, you can write it. Go do your monitor
manufacturer site and get the specs.