Client-Mac OS X ?

OS X Server does support NetBoot, which would do what you are asking. It does not dedicate a core to the client. You would also probably be best off using a dedicated server for this task. In other words, you wouldn't be lending spare cores to the clients. The server would host images that the clients would boot and run from, and you shouldn't expect to use the Mac Pro as your personal Mac and use it as a server at the same time. Overall performance might be OK (or not) but the server OS is not the same as the client OS, so once you install OS X Server on a Mac Pro you should expect that some apps you want to use and would run on a client OS would not work as expected.

I think the Windows software is called 'Terminal Services' all clients are 'Thin clients' and behave just as you describe.

I do not know off any similar service on OS X server,the Netboot feature requires a full client 'iMac etc' and loads a full version of OS X modified for you infrastructure. All apps run on the local machine not on the server, but home directories and shared directories could be on the server.

I could be wrong but that is what I understand.

guess netboot isn't exactly what I was searching.

Netboot would be how the software bits are loaded into the target box.

Want that my Mac Mini's hardware doesn't process anything, it shall not even have a harddrive (unless its required for a thin client software)... The Mac Pro shall do everything.

What you are describing here was once known as a classic "dumb terminal" design where a VT100 terminal connected into a central VAX computer via serial terminal line, and later as an X Window display, and more recently as a "thin client" or other such; to something akin to a SunRay terminal, or what Microsoft calls Terminal Services, or a [VNC|http://the.taoofmac.com/space/VNC] display.

Apple doesn't do this configuration particularly well, save for loading Mac OS X client software down into the box and operating as individual hosts. Possibly with OpenDirectory and related authentication to control access and options. Here, I'd probably look around for true" "thin client" terminals here, and use the Mac boxes to run Mac OS X software. Full-on Mac boxes are comparatively expensive, as "thin client" boxes and terminals go.

The most direct analog here is Aqua Connect Terminal Server, a commercial product.

Or VNC or ARD.

Or screen sharing, with Mac OS X Server running on your Mac Pro box. Here, you can connect in via Screen Sharing. (Available via Server Admin.)

Its quite like virtualization I guess

Virtualization is entirely different technology, at least in the local vernacular.

There is no thin client Mac. Anywhere. The closest that you will get to WTS is AquaConnect Terminal services. There is no way for you to provision one of the eight cores on your Mac Pro to be used by a TS or Netboot client. It just doesn't exist -for now. To do what you wish to do with Apple hardware and OS X, Netboot is the only way to do it. Period.