Installation of minimal selection of Centos 5 – only base, core and dialup selections, will leave a small-sized system, with the minimum required.
This is a good setup to start setting up a firewall, and it lets you add any required package later using ‘yum’.
This said, you cannot forward X over SSH at this state. A minimal setup is a minimal setup, after all. I was searching for the package required to allow X forwarding over SSH, and found it – xorg-x11-xauth
Install this one, and X will be forwarded over your SSH connection (you still need to connect with the ‘-X’ flag from the client).