A whole X session or just a single app? You can do a single app just by making sure "X11 Forwarding" is set to yes in client's /etc/ssh/ssh_config and the server's /etc/ssh/sshd_config. But if you want to run the entire session over the wire, you need to use XDM/GDM/KDM. Check out the XDMCP HowTo at TLDP and thoroughly read the config files for your display manager.