there are some mac emulatros for mac os but all of them are for mac os 9.x or earlier - all those i saw were indeed for 7.1 to 8.6 and that is pretty old and not what you want (OS X is very different)
more about darwin here:
http://developer.apple.com/darwin/
it does not promise on which systems it does run. i was a few years ago wanting to install darwin on a pc .. ended up playing with a lot of linuxes instead, and once i had saved enough money for a mac, just got a mac (it felt like it was what i wanted).