Well, my suggestion may be a bit of stretch but you should try
Ubuntu or Kubuntu.
My main point comes from the fact that changing themes on Windows
could lead to lack of stability. Secondly, if you decide to choose Kubuntu,
you will have places like
www.kde-look.org where there
are OSX themes available.
I would vote for Vista, but you may need to expend some $
getting a nice video card.