Gotsouldr, if you plan to buy a new Mac, then it will come with the "Leopard" OS. If you have an older Mac or plan to buy an older, used Mac, then it may come with Tiger or Panther. Regardless, you should be able to run the latest iTunes with it. Mac OS older than Tiger will not support "Front Row" remote-control capability (there is a workaround).
Sound-wise, I do not believe there are differences between late model Mac OS'es.
Sound-wise, I do not believe there are differences between late model Mac OS'es.