Alienware, the gaming side of Dell, has begun selling gaming rigs that come with Ubuntu pre-installed. These Windows-free machines start as low as $599, and they are actually decent builds for the money. The rigs should be able to handle all current games without issues, and the higher-end models will be able to handle all of the next-gen games that are coming out soon.
Alienware's explanation for gaming on Ubuntu revolves around Steam being on Linux. The company cites that there are "over 25 gaming titles available and more being added." While the selection of major releases on Linux is still small, it is growing as time goes on. These machines are definitely for the early adopters, but if all else fails you can always install Windows later.