I wonder...

OS X is a tru OS, not a desktop Manager
Aqua is the Desktop Manager that Runs on OS X
exampleBSD, FreeBSD, AIX, Linux, Solaris, OS X are all Unix Based Operating Systems

KDE, GNOME, XWindows, Enlightenment and Aqua are all desktop managers, the gui that interacts with the os so you don't do everything in command line.

You can compile X windows to run on OS X which means in theory you can also compile KDE and Gnome (not that you'd want to, to me they remind me of winblows)

Now take into consideration I may be wrong, but thats how I understand it.
what component would Quartz be, then?

also, in theory, couldn't Aqua/Quartz be compiled to work in an X86 version of xNIX?
Quartz is a layer ontop of aqua, almost like a postcript aplication always running.

I don't believe you can compile Aqua on x86 machines because of all the proprietary includes you would also have to compile, where as the others that I listed are open source and the libraries and includes are pretty standard on both architechtures (if you installed developer tolls cd)

"XWindows" (as you called it. it is really X Window System, X11 et cetera) is not a desktop manager. it is a windowing system. the most common consumer implementation of this is XFree86. on top of that you run a window manager, which could be twm, enlightenment, et cetera.

desktop managers include KDE, GNOME... which in turn need window managers (KDE's kwm, GNOME's default manager sawfish (right?)) to be useful.