yes, enlightenment is very nice. I don't know if anyone's got it running under Darwin X11 yet... people seem to have fvwm, fvwm2, windowmaker, and perhaps ice posted on http://www.darwinfo.org.
(They have great FAQs and other stuff about running X11 on darwin and on OS X boxes. check it out.)
As far as fullscreen, VNC clients can do that passably, but you're right, starting X directly is nicest. (Useful hint, thanks to AppleWatcher: To return to aqua after you've logged in as >console nad finished with the X session, just type "exit" from the login shell and it automatically boots you back to the loginwindow.)
First, a terms clarification: OS X runs a window server / window manager that many people call "Aqua." X11 is the UNIX window server, different than OS X, but it can be run side-by-side with OS X
To answer your question: sorry, if you get Xfree86 (an X11-compatible window server) with drivers for your accelerator, those drivers will only work within the context of a native X11 session, and will not cross over to accelerating your OS X "Aqua" sessions. X11 is good if you have ported programs from UNIX that require it, like the GIMP, but X11 is totally unable to run Aqua programs. If this doesn't make sense, I'l try to clarify more.
(Does the new version of Xfree86 finally support all the ATI cards, anyway? I profess ignorance.)
Understand that OS X is a beta, and Apple has long told us that the beta would only support accelerated video from the rage 128. All the drivers will come with the 1.0 release.