FROM: http://www.trap17.com/forums/Widescreen-Resolutions-Ubuntu-704-t51326.html

... you can manually edit what resolutions are available in
the Xorg.conf file. Modify it so you add a line like this:

Modes       "1680x1050" "1280x1024" ...

... to get 1280x800 working on my laptop,
I had to change xorg.conf to have the following section:

Section "Screen"
Identifier "Default Screen"
Device "Intel Corporation Mobile 915GM/GMS/910GML Express Graphics Controller"
Monitor "Generic Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1280x800"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x800"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x800"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x800"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x800"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x800"
EndSubSection
EndSection