Ok the reason why you would get that message about this file cannot be loaded to this machine is because as far as I am aware - you cannot upgrade from Snow Leopard to Yosemite directly (from the Mac App Store). You would first have to upgrade from Snow Leopard to Mountain Lion (or Mavericks) and then upgrade to Yosemite. Although I am happy to be proven wrong and corrected on that
You might also have trouble with getting your graphics acceleration (for video playback and games) to work with a GTX285 under Yosemite as it is quite an old card nowdays and may not be supported under Yosemite
Also a quick check of the Mac App Store just now seems to indicate that the Mavericks install download has been taken down from the front page and a quick search on there seems to not find any trace of the Mavericks installer on the site anymore.