In case the frame buffer size is a limitation for the resolution you want to add (it happened to me), just modify/create the "Virtual" parameter in the "Display" subsection of your xorg.conf. Here's how mine looks after configuring the frame buffer size to use my 22' widescreen (resol: 1680x1050)...
Code:
Section "Screen"
Identifier "Default Screen"
Monitor "Configured Monitor"
Device "Configured Video Device"
SubSection "Display"
Virtual 2960 1050
Depth 24
Modes "1680x1050" "1280x800"
EndSubSection
EndSection