I would hold off on buying that Steam Machine until you can find out more about its specs. One prime missing item in the article you provided a link to is how much RAM can be installed on it. I read somewhere (and I can no longer find it) that GC3 would require a minimum of 4G RAM. If accurate, that implies that 4G would support only the smaller galaxy sizes that GC3 will have available. But would the Steam Machine come with even that much?
And what about the CPU they are including? They don't even say what model AMD CPU they are using for their machine, so it is hard to tell if it will support 64bit addressing, which StarDock says is a design point for GC3. I think the chances are good it will, but I suggest finding out for sure before going down this route.
I was able to find out that the GPU specified as coming with this machine will have 2G memory and is DirectX 11 compatible. Is 2G enough? StarDock hasn't said anything about this. At least the GPU will meet the DirectX requirement they have specified (minimum of DirectX 10).