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.