This is a short post on how to install the necessary components to get X Server running on a bare bone RedHat 6 Enterprise Server (CentOS6) installation.
The following commands should be run from a console window as the root user.

Install barebone X support

# yum groupinstall "X Window System"

Install X and the Gnome Environment

# yum groupinstall "X Window System" "GNOME Desktop Environment"

Install X and the KDE Window System

# yum groupinstall "X Window System" KDE

Install the XFCE desktop environment

# yum groupinstall "X Window System" XFCE

When you have chosen your desired desktop environment, make the final change to make the Gnome Display Manager show you a logon screen.
Edit the file /etc/inittab and change the line
from

id:3:initdefault:

to

id:5:initdefault:

You will be greeted with a graphical logon screen the next time you have rebooted your server.