This from Mac Singularity.
X11 is a windowing system for UNIX, but a very rudimentaryone. The idea is that a unix application can communicate to the window ?server? and say ?draw a window? or ?draw a line? or very basic commands like that. And then, X11 draws whatever it?s told. In order to make X11 have some sense, a program called a ?window manager? keeps tabs on all the various windows that have been drawn.
Unless you are planning of delving around in Unix, you do not need to install X11.
The Photoshop question? I would take a look at the system requirements for this application and make a decision from there.
Opening the envelope, and the requirements are:
owerPC
Hi guys, i need to ask a question, what is X11?
What is it function on OS X?
Other thing, the new macbook's have a new graphics cards and i heard that the new Photoshop CS4 will have something to do with the new graphics cards. my question is: will the new CS4 work with the old macbooks?
Tks guys.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic