Here how I do it:

$cat  ~/bin/setdisplaymode 

#!/bin/bash

#

if [ -z $1 ]; then
        echo "Usage: $0 mode, e.g:"
        echo "$0 1920x1200"
        echo "$0 1680x1050"
        echo "$0 1280x1024"
        echo "$0 1280x960"
        exit -1
fi

MODE=$1

# find a connected monitor
#
MONITOR="`xrandr --listmonitors | awk '/0:/ {print $4}'`"

if [ ! -z $MONITOR ]; then
        echo "Setting $MODE mode on $MONITOR"
        # set the resolution
        xrandr --output $MONITOR --mode $MODE 2>&1 >/dev/null
else
        # try to set the resolution using some standard monitor names
        echo "Trying to set $MODE mode..."
        xrandr --output DVI-I-1 --mode $MODE 2>&1 >/dev/null
        xrandr --output DVI-D-0 --mode $MODE 2>&1 >/dev/null
fi