Hmmm. Maybe you might want to look at Linux? When it comes to actual work -- non-video, that is -- it is more than enough, it is totally free, and will run on anything from embedded systems to mainframes. Needs a bit of hand-holding, though, and sucks on laptops, which is what is driving a lot of the Linux people to MacBooks (the reason I switched, actually). Still, depending on what you do for a living, you might want to take a look.
This is actually relevant to the discussion because not only will Valve support OS X versions out of the box, there are more and more rumors of
a Linux version. If this is true (a big "if" at the moment), then they have been very clever with their tool chain. This would mean that you simply don't have to care about the operating system, which is taking a page from Google's philosophy.
Will this be relevant to sales? Well, remember you'll buy one copy of the game and you can download it on as may computers as you like without having to care about the operating system. So taking about "Windows sale", "Mac sales" or (maybe) "Linux sales" becomes moot. They just sell system agnostic games. It's like asking if Google Mail will work on Linux or OS X.
What this would do is cut down the minimal total cost of ownership for a machine that only plays games beyond anything that Apple or Dell or whoever could dream of offering. Buy exactly the parts you want for the lowest price you can find, install Ubuntu as the OS (free), ignore anti-virus software (an additional cost for Microsoft operating systems), and start gaming.
Again, Firaxis is competing with Valve for time, attention and game budgets, if not (currently) directly going head-to-head for individual games. The easier it is for me to just drop into a game from my work setup, the more I will play it (the "FarmVille effect"). It would be nice if Firaxis could make it just as easy to play Civ V on any machine and any operating system. This whole thread would never have been necessary, for one thing. And if Valve can do it, it can't be magic.
Though of course nobody would ever get any real work done in their lives ever again

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