I think it could be added in one of two ways. You could try either: gconftool-2 -g 1680x1050
or modify this part of your xorg.conf (back it up first, just incase)
Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
SubSection "Display"
Depth 24
Modes "1680x1050"
End SubSection
EndSection