Try Ctrl-Alt-[+] ... using the + from the number pad.
Now, if you edit the conf file you will see a screen section. Find the display with the depth you want and add the resolutions you want (the default one will be the first listed).
Viewport 0 0
Modes "1024x768" "800x600"
Also, near the top of the screen section, (under Monitor) add the line
or whatever you want the default to be.