Use Linux and add Steam - there are a vast number of games available for it as the "Steam OS" itself is linux-based.
Grab a copy of CentOS and add the required Steam stuff, as per the instructions here:
http://negativo17.org/steam-for-centos-rhel-7/
Or grab a pre-built SteamOS image and install that:
http://store.steampowered.com/steamos/
Using Centos gives you a more flexible general-purpose machine for running general programs on, the pre-built system is simpler for a dedicated gaming setup.
If you want to use centos and give it all possible multimedia capability, follow the instructions here:
https://wiki.centos.org/TipsAndTricks/MultimediaOnCentOS7
(It's primarily a high-reliability business system so does not automatically include all the media player type stuff that you may want on a general machine - but everything is available and easily installable as above).
Get the Centos system itself below: