Why? Because not only do you get the newest grade processor, newest model of ram you also get a more broad variety of games to play. Tech Inferno forum user Kloper has put together a rather in-depth guide on building an external GPU setup which connects to the MBA via its high-speed Thunderbolt port. It would be far more beneficial to you to go out and buy a top of the line PC like an Alienware desktop then to spend 600 dollars on the top of the line graphics card + 200 for the external graphics card enclosure box.
Why? Because majority of the games are programed to have both a left and right click feature, something that is clearly not functional in the Mac OSX more so if you have a mac branded mouse. Native games on Mac computer's are slim and scarce and have a clear lack of a wide verity of games. honestly your better off building a gaming desktop for the 1200. so your card has a pretty bad bottleneck to deal with. thunderbolt 1 runs at 10gbps, a normal pic-e x8 lane is 64gbps.
So when buying go ahead and add an additional 200 dollars to your initial price of your card you are going to buy. gtx980ti, the external enclosure will cost you about the same, you'll have to make sure they support thunderbolt 1, and it's a far from perfect solution. You can face some driver struggles depending on the brand of card you get.Ä«uying your first graphics card will be more expensive then it would be on a traditional PC because you will have to buy the additional device to attach your new graphics card to. You will have to buy an extra component which can cost anywhere from 150-200 dollars to hook your graphics card onto and attach it to your mac. There is nothing wrong with an external graphics unit, it does have a slightly less then average performance on mac computers then it does on traditional PC's.