"When" sounds more natural to me. If we were to use once it might be more like "Once beijing has hosted...." However, if you were to say "Once Beijing hosts..." no one would look at you funny. It sounds OK.
Both are fine, but I agree with James. "When" seems more natural. Also, "once" sounds "only when". The sentence seems to be telling me that only when Bejings hosts the olympics will the world find a city and country that has been transformed. Maybe that's just me.