Be root (using su
if at all possible) and do:
# dpkg-reconfigure xserver-xfree86
When asked for information, fill it in to the best of your present knowledge. Most important are graphics adapter and monitor specs - try to get as close as you can to the values you want and be as precise as possible with the information you provide.
While you could edit your XF86Config-4
by hand, the procedure described above is the one you are meant to use.