Like most things in Linux, there are several different ways to do this. The most basic: edit the config file. Since we don't know what version of X you're using, here are the most common files (you'll probably only have one):
As root, open whichever one of these you have in any text editor and search for the string 'DefaultDepth'. Underneath that will be several 'SubSection "Display"' sections. Find the one that matches DefaultDepth. You'll see a list of resolutions on the 'Modes' line. Add the res you want to the beginning of this list, and restart the X server. You can also add your new res to all the Modes lines, so it will be available in all colour depths. WARNING: make sure that you don't put in a res that your system doesn't support, or X Windows won't work at all.
Alternatively, there are some GUI tools that can do this as well. In KDE, open the Control Center and go to Peripherals/Display. This will list all the available resolutions and let you change to the one you want. There's one for Gnome too, but I don't remember what it is offhand. Good luck!
Last edited by MidnightCmdr; 12-29-2005 at 10:45 AM.