Aqua is more than just a window manager - it's the entire windowing system, more akin to X11 itself than a particular WM or desktop environment. It's possible to install Apple's X11 and from there Enlightenment, or whatever you fancy...but doing that pretty much renders the OS useless. It won't run apps designed for OS X (Carbon or Cocoa), only things that have been built to run under X11 in the first place. Literally nothing that runs on OS X will run under X11; it's not a Mac anymore but rather a Unix box.

If you still want to go for it, instructions for 10.4 are here with issues on 10.5 discussed here, or you could check out PureDarwin - they're doing essentially what you're looking at by packaging the open-source stuff from Apple. They also have MacPorts up and running so you have a decent selection of X11 software.

So in other words, I won't be able to run Aqua apps at all, even with all their libraries and everything else still installed correctly, if install X11 and reconfigc? I can't believe it, but it almost sounds like running X11 on a Windows box is easier than it is on a Mac...and it's probably more worthwhile to do on a Windows box too.

How annoying is that?

Apple provides a complete X11 environment, you can install all the Fink and MacPorts stuff you heart desires. I run some stuff myself that's been ported to OS X but not written for Aqua (e.g. Wireshark).

But as a replacement, no. You categorically, absolutely, unequivocally cannot run OS X apps under X11 - Aqua is the entire end-to-end display system; it's the display server, window manager, 3D system, and everything else all rolled into one. It's actually a pretty elegant modular system (Google up Quartz and CoreImage), but at any rate, the whole package is indispensable for running Cocoa apps. They simply don't make X11 calls.

If you just want to change the look, there may be themes available, but as I'm already late for work you're on your own

You can get X11 programs to talk to Cocoa, but not vice versa. If you want to change the behavior of Aqua, there are apps for that, but it's still going to be Aqua.

If you're just going to be running stuff from Fink (or wherever you get your packages), just switch to root screen X11 and you'll be fine. If you need to run Aqua apps, you're stuck with Aqua's window manager AFAIK.