I am sitting in front of apollo and launch programs on zeus without SSH: xhost +zeus; export DISPLAY=apollo:0.0.; after you are done type xhost -zeus.

SSH can forward X connections and there's no need for the above steps

GDM: to enable network connections change DisallowTCP=true to DisallowTCP=false
KDM or XDM: both on settings in the Xservers file. Look for a line that begins with :0. This line contains a command that KDM or XDM uses to launch the X server. If this line contains the string -nolisten tcp, remove that string from the line.
X launched from text-mode login: if you log in using text mode and type startx to launch X, you may need to modify the startx script itself, which is usually stored in /usr/bin. Search this script for a string -nolisten tcp. Chances are this string will appear in a variable assignment (such as defaultserverargs) or possibly in a direct call to the X server program. Get rid of the string.
SSH: /etc/ssh_config set ForwardX11 to yes, /etc/sshd_config set X11Forwarding to yes.