Hello fellow *nixers,
In this tutorial I'm going to explain how to run X programs from a remote computer.
This is an already well known subject for most *nix users, however this might be useful for reference purposes.

If you want to run remote X applications, like GUI, on your local machine, this means running them inside your current X session, you'll need to follow the following steps.

Make sure that SSH on the remote machine, the one you are not currently sitting on, allows X forwarding (Obviously the ssh deamon should be running also).
Open /etc/ssh/sshd_config, on the remote box, and make sure that this line is not commented.
Then all you have to do is to start a new ssh session from your machine with the -X option.
If you run into problems here just restart the sshd.
The -c option allows you to choose the encryption. blowfish is said to be fast, but remember that this doesn't mean that it is safer.
The last step is to simply run the X program from the ssh session you just opened, as if it was a local terminal.
You'll notice that the programs will open inside your current X session as if they were running locally.

On the other hand, if you want to run a nested X session inside your current X session, then you'll need to proceed as follow.

Install Xephyr from your favourite package manager.
Xephyr is an extension to the X server that allows you to run an X display inside the one you are running, which is exactly what you need.
You'll also need to be able to forward the X session so check the previous method to see how to do that.
A little parenthesis about the $DISPLAY environment variable.
This variable represent the current display the X session outputs on.
This means that the current X session is using the display :0.
What you need to accomplish is to be able to redirect the output from the ssh X program into Xephyr nested X session.
Thus, run the following command to start Xephyr and make it listen for programs that needs to be displayed on the :1 .
This will also make it run in the background and throw error messages into /dev/null/.
You can change the definition, in the screen parameter, according to your preferences.
Now, in the same terminal run the following command to be able to use the :1 display for the next commands. (This might differ depending on the shell you are using)
Finally, simply run the ssh session.
Change the window_manager to any program you want to run inside the nested X session.

This should cover most of the remote usage of X programs.
I hope this is now as clear as water for people that were looking for an answer on the subject.

That's all folks!