Yes, you can put Linux on the comp in your sig, however, the newest core that most Linux environments use (red hat, fedora, debian, etc.) say Athlon has received more buggy performance than Intel.
OtOH, You can't use Mac OS's with an AMD processer (or an Intel).
However, if you just like the layout of the Mac you saw, there are plenty of Mac skins on the internet to make it look as if you were running a Mac.
One last thing, Linux (and windows, but not as well) can run Virtual machines of any OS, so you can run a Windows (Works, I use it) and Mac (I believe, I've never tried).