Given a Linux distribution with XFree86, it may be desirable to use an Xserver other than the one provided by the distributor, for instance if a card is newly or better supported with a new release of XFree86. Because a distributor who packages XFree86 does the X server and all the other stuff on which many things depend, proper packaging is a time consuming process which may take a while.
When there are no major changes, it is possible to change the Xserver without touching anything on the client side, and even most of the Xserver support files (fonts etc).
This document describes the process for a specific set of circumstances but should work with most versions/distributions:
It's very simple really. You basically get the binaries from xfree86.org and replace your server with it.
That's it. Now startx -- -xf86config file-with-right-ModulePath should work. If you have changed /etc/X11/XF86Config with the module path, startx and other programs running the Xserver should work without further parameters.
At some stage before the release of Debian 3.0, xdm stopped working with this scheme. Other login managers like wdm worked fine. It seems the problem is due to the XDM authorisation scheme not being compiled in the server compiled at xfree86.org. The solution was to add the following line to /etc/X11/xdm-config:
DisplayManager*authName: MIT-MAGIC-COOKIE-1
This (probably) makes XDM use the standard MIT authorisation method.
(While looking at XDM, I also found out that /etc/X11/xdm/Xresources can be tweaked to restore the traditional minimalistic 2D look of XDM instead of the horrible '3D' ugliness introduced lately. The entries are reasonably self explanatory.)