Its Mac, as in short for Macintosh. Not MAC as in Media Access Control (for ethernet cards).
You can't run a Mac OS on a regular PC without violating the OS X TOS. There are ways to do it, certain hardware produces better results than others. Even a company with a plug in workaround.
But to run all 3 on 1 set of hardware legally you'd need to buy an Apple branded computer with an Intel processor, then use Bootcamp to install/run Windows XP/Vista or some variety of Linux.
At one time recently I ran OS X 10.3.9, 10.4.10, 10.5.x, Windows 2000, Windows XP, Windows Vista, and Ubuntu 8. I've slimmed that down to 10.5.5, Windows Vista, and Windows 2000.
As far as speed of the OS, I think that is made a much bigger deal than it really is. Unless you are gaming or doing some serious video/photo editing a slower computer doesn't hurt you much. I'm running Leopard (10.5.5) on a G4 450 (had to modify some files to get it to install on that slow of hardware) and yes its a bit slow, but I could do word processing, spreadsheets, IM, and browsing on it just fine if I had to.