The symbolic links are needed for properly starting several XFree instances, as well for properly exiting an X session. This applies for both starting X from console and the automatic starting of X by the display manager (kdm, gdm, xdm).
You need to create as many symbolic links to the modified X server binary (or to the original X server in case you do not need a modified one), as the number of your video cards/X sessions.
I assume that you will have to use a modified X server, but in case you do not need it, use the following commands to create the links to your original X server:
cd /usr/X11R6/bin/ ln -s XFree[modified] X0 ln -s XFree[modified] X1 ln -s XFree[modified] X2
In case you use the provided rpm packages, you'll only need this if you want more than 4 parallel running X servers/X sessions, as the rpm creates 4 symbolic links to the X server binary.