Linux xrandr add new mode

Run cvt and give resolution as params:
$ cvt 1280 1024

Cvt prints out mode line for xorg.conf. Paste this line as an argument for xrandr:
$ xrandr --newmode "1280x1024_60.00" 109.00 1280 1368 1496 1712 1024 1027 1034 1063 -hsync +vsync

Add new mode to list of available modes:
$ xrandr --verbose --addmode VGA1 "1280x1024_60.00"

Set new mode for appropriate output:
$ xrandr --output VGA1 --mode 1280x1024_60.00

Resources:
UbuntuForums
AutoStart of dual monitor mode: ThinkWiki