This article refers to Ubuntu 9.10.
If you can't see a resolution option of higher than 800x600 this probably means that Ubuntu has not recognised your graphics card when the operating system was installed. The main group of cards that are not detected properly are Nvidia Geforce cards running under AMD systems.
To fix this go to settings and then hardware drivers. Ubuntu should search for appropriate drivers and install them. You will be asked to activate the driver and then reboot. Now when you go into the display settings you will be told that Ubuntu can't control these drivers do you want to use the manufacturers software? You select Yes and it launches the Nvidia driver interface where you can set the resolution to all of the ones that are possible. You should also see your monitors correct name showing (assuming it has been able to detect it correctly via its plug and play identifier).
I can't guarantee this will work for you, but it has worked for me and its a lot easier than many of the solutions I have seen posted in forums. Its probably better to try my method before delving into the operating system at the terminal window.