How to connect to other Ubuntu machine as X Server?

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http://askubuntu.com – When I want to work on my local Linux machines from windows, I use Putty+Xming and I set the Xming port in Putty connection settings. This way I have a terminal on the machine and if, for example, I run the command 'firefox' a firefox window is opened in Windows (while it is really running on the Linux machine). I want to do the same thing from other Linux machine (they are both Ubuntu machine) connect by SSH and become this connection X Server so any graphic component will be opened on local machine. How can I do that? (HowTos)