SteamOS does not support APUs afaik. You'd be better off getting an Athlon 750K and a 650 Ti or similar card. SteamOS also does not support AMD Graphics Cards last I checked.
Here's a couple of alternative parts for your build. Better PSU, Better Hard Drive and an Athlon 750K instead of an APU. http://pcpartpicker.com/p/3d3RG
This is kind of changing from a steambox its more going to be a basic home pc with some gaming so maybe an apu pc will be better like this one http://pcpartpicker.com/p/3dbdN