One doesn't have to have an expensive computer to run Mac OS. Recently I just switched my netbook from Mac OS Snow Leopard. Operating systems like anything else that is software are a bit of a subjective choice.
Mac OS works fine on my MSI Wind u100 but, I never really liked the interface as much as my normal linux setup (openbox, tint2, wbar,xcompmgr). Being able to run things that have official ports was nice though.
Truth be told, I have never had a problem running any major operating system that wasn't Windows with regards to viruses. Ecomstation was nice, and ran 16-it windows programs. Solaris was good, just not great hardware compatibility, BSD seems to work as well as linux, but there does seem to be less hardware that works with it as well.
In any case Linux Mint, (ubuntu based distro with nonfree codecs installed by default) MEPIS, (debian based, no beta software) or Mandriva are really peoples best bet for a cheap operating system that is user friendly and resistant to all of those viruses and spyware that seem to plague the Windows world.