If your preferred resolution is not listed under System -> Preferences -> Display , you can add it by hand.In this example we will add the resolution 1680×1050 to the notebook display (LVDS).

At first you have to create a modline using cvt. :

cvt 1680 1050

This will produce something like

# 1680x1050 59.95 Hz (CVT 1.76MA) hsync: 65.29 kHz; pclk: 146.25 MHz
Modeline "1680x1050_60.00" 146.25 1680 1784 1960 2240 1050 1053 1059 1089 -hsync +vsync

We copy everything after Modeline and add the resolution using xrandr:

xrandr --newmode "1680x1050_60.00"  146.25  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync

xrandr --addmode 1680x1050

Now we can change the resolution with xrand:

xrandr --output LVDS --mode 1680x1050

In order to make these changes persistent, you can add a little BASH-script.
Create ~/.xprofile and paste your xrandr options.

#!/bin/bash
xrandr --newmode "1680x1050_60.00"  146.25  1680 1784 1960 2240  1050 1053 1059 1089 -hsync +vsync
xrandr --addmode LVDS 1680x1050

Make the file executable with chmod a+x .xprofile.
The script will be executed during start of the xserver. You can change the resolution now in your Display dialog.