For *my* suggestion: (doing this from memory - I'm at work ona Windows machine)
(Also note, I haven't verified this works the way I expect it to. But logically, it should.
Log in as normal.
Open a Konsole Window (should be an icon on the left side of your task bar at the bottom. Or search through your menus.)
In that Konsole window, type "kdesu kwrite" without the quotes.
This should pop up a box asking for your root password. Enter it, and hit <Enter>.
KWrite should start. -- The *owner* of this KWrite session is root!
Click the File -> Open button on the menu.
Navigate to the "/etc" directory. (Maybe it's "/etc/X11", I don't remember.
Select the file "XF86Config" (or xorg.conf, or whatever is correct for your system) and open it.
Edit the file, and save it.
Close the Konsole.
If there are errors in the file, your gui won't start. In that case, log in as root, type "cd /etc/X11", delete the file you just created, and rename the backup file (XF86Config.~) to the correct name, and start over.
Edit: Gee, and after I typed all these nice instructions, too.