Provided you selected the proper video card at install time, you should have the proper X server installed. When later running Xconfigurator or xf86config, you need to make sure you select the same video card or the autoprobe will fail.
If you think you installed the wrong X server for your video card, you will have to install the correct one before it can be configured. For instance, if the CD is mounted on /mnt/cdrom, and you need to install the S3 server, enter the following commands:
cd /mnt/cdrom/RedHat/RPMS rpm -ivh XFree86-S3-3.1.2-1.i386.rpm ln -sf /usr/X11R6/bin/XF86_S3 /usr/X11R6/bin/X
This will install the S3 server and make the proper symbolic link.