You don't need to install any other operating systems, if all you want is to be able to run KDE on Mac OS X. You said you're running Apple's X11 v.3. The only other thing you need is a program called "fink". You can get it here:
http://fink.sourceforge.net/
Once that is installed, run (in the Terminal):
[code:1]sudo dselect[/code:1]
or, if that doesn't work,
[code:1]sudo /sw/bin/dselect[/code:1]
Then, choose "Update", then "Select" Find the Base KDE 3.1 package in there, and select it for installation (sorry, you'll have to figure out the details on your own, there's help in the program).
Once it's installed, open up the X11 program, and type:
[code:1]/sw/bin/startkde &[/code:1]
And there you have it. I've got a couple of bugs in my installation of it (such as the sound not working...), but hey, it works.
Hope this helps,
Ben Foster
MacOS X user!!