The XF86Config file tells the X server what kind of monitor,
video card and mouse you have. You must create it to tell the
server what specific hardware you have.
XFree86 3.2 introduced a new, user-friendly configuration utility called XF86Setup. It has to be run as root. Refer to its documentation for details about its use.
You'll need info on your hardware:
The recommended way to generate an XF86Config file is to use the
XF86Setup utility. The xf86config text utility is still there
for the (few) cases where XF86Setup can't be used. Also, there is a
sample file installed as /usr/X11R6/lib/X11/XF86Config.eg,
which can be used as a starting point.
For details about the XF86Config file format, refer to the
XF86Config(5) manual page.
In order to protect your hardware from damage, the server will no
longer read XF86Config files from a user's home directory,
but requires that it be in /etc/XF86Config,
/usr/X11R6/lib/X11/XF86Config.hostname or
/usr/X11R6/lib/X11/XF86Config.
Once you've set up a XF86Config file, you can fine tune the video
modes with the xvidtune utility.