An American company has indeed boxes that can contain an ExpressCard card or any PCI-Express card. But the bandwidth just isn't there yet.An external graphic card on a Mac? Some dream of. I too was very excited when Thunderbolt came out and dreamed about eGPU and perhaps one day we can buy TB display with proper desktop GPU built into them. Obviously you will get less responsive experience but they are continually improving the technology and it isn't all that bad as long as you aren't playing FPS games competitively. And with the Windows PC, use Steam's In-Home Streaming to play those games on your Mac. You can build a Windows gaming PC and stick it to wherever your router lives (and use ethernet connection between the two for better latency) that way at least you won't have to mess around with two machines on your desk. There is one other option available as long as you are ready to make some sacrifices. Plus you can't avoid having second display since eGPU can only be hooked up to external display. eGPU simply isn't efficient with current technology(and too expensive to the point where you are better off buying another PC which is why there are so many suggestions for that route) and you will be much happier with a second machine if you care about gaming experience enough that you are thinking about eGPU solution. It's tried and true fact that Macs aren't as efficient when it comes to gaming no matter what kind of peripherals you buy.

It's quite frustrating to use a Mac if you are a gamer and you'll likely to get similar answer anywhere else, regarding eGPU and there is a very good reason why people suggest second machine over eGPU despite they are Mac users/lovers themselves. Xeon, ECC and workstation GPU isn't built for gaming. Click to expand.There are no real benefit of going MacPro for gaming.

External graphics card for imac