First individual resolutions have to be set up in the /etc/X11/XF86Config file; then you can toggle among the valid resolutions for the active display using Ctrl-Alt-"Keypad +" or Ctrl-Alt-"Keypad -" .
Originally posted by Stefan Wagner: /etc/X11/XF86Config-4 should be the current config-file to modify.
Well, no. The rule is that if there's a XF86Config-4 file, then it's used; otherwise the XF86Config file is used. Unless you're running both XFree 4.x and 3.x on the same machine, there's no reason to have the oddly-named "-4" file; it was just a temporary, stopgap compatibility measure.