How to xclient and xserver through LAN?

Hello,

I am interested in installation of system with X application server and X terminal computer under Linux.
I will get powerful server runing linux. The things I want to build are to run an X application (eg: firefox) on the powerful server (with 3D acceleration)and use my old desktop as just a terminal. All works via ethernet 100mpbs. I specify that runing a local application must works too.

The desktop are neither thin client nor diskless. It is just PC under linux with GNOME or KDE.

First question: could someone tell me how to set up linux on the server to send and listen x11 data through TCPIP / LAN ?

Second question: how to config users on the server because I plan to work with many terminal and many people, not only me.

Third question: how to tell my old desktop PC to listen and send x11 protocole through TCPIP/LAN ?

Any http links and case studied in the past are welcomed.

Thank you.