copied from: http://ubuntuforums.org/showthread.php?t=1112186 worked on ubuntu natty (11.10)

Here are the steps

Use xrandr to make sure that the new mode can fit within the maximum framebuffer size

  xrandr | grep maximum

Use gtf to create a mode line (gtf <pixel horizontal> <pixel vertical> <frequency>)

  gtf 1440 900 59.9

Add new mode using xrandr (xrandr –newmode <name of new mode> <output of gtf-command>

  xrandr --newmode "1440x900_59.90"  106.29  1440 1520 1672 1904  900 901 904 932  -HSync +Vsync

Add this newly added mode to the desired output (VGA/LVDS etc)

  xrandr --addmode VGA 1440x900_59.90

Choose the new mode

  xrandr --output VGA --mode 1440x900_59.90

  sudo gedit /etc/X11/xorg.conf

Find this: (yours might look different than from below) Section “Screen” Identifier “Primary Screen” Device “ATI Technologies, Inc. M22 [Radeon Mobility M300]” EndSection

and add this to it

DefaultDepth 24 SubSection “Display” Depth 24 Modes “1280×1024” “1024×768” “640×480” EndSubSection

so it should all look similar to this:

Section “Screen” Identifier “Primary Screen” Device “ATI Technologies, Inc. M22 [Radeon Mobility M300]” DefaultDepth 24 SubSection “Display” Depth 24 Modes “1280×1024” “1024×768” “640×480” EndSubSection EndSection

MAKE SURE YOU ADD YOUR NEW RESOLUTION TO THE MODES LINE Modes “1280×1024” “1024×768” “640×480”

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: 1680×1050)…

Section "Screen"
	Identifier	"Default Screen"
	Monitor		"Configured Monitor"
	Device		"Configured Video Device"
	SubSection "Display"
		Virtual	2960 1050
		Depth 24
		Modes "1680x1050" "1280x800"
	EndSubSection
EndSection