So it seems first of all that beyond earth (the new civ game) will be official released somewhere in september, a clear date has not been given yet but judging by the last release dat of 10 September or Civ 5 it will probably be released again early in September.
Then most likely it will be released a bit later for Mac most likely a month later. For OSX the likelihood that it will include snow leopard in the system requirements will in my opinion be quite low.
For Windows it is hard to say but I think that Vista will also be dropped for the new Civ Beyond Earth game.
So in the end my prediction with using facts is:
Windows Release: September 10th System Requirements Windows 7 and higher
OSX Release: October 22nd System Requirements OSX 10.7 and higher