The Steambox is a small form-factor system. There would be no sli or Xfire setup with the limited space, and the board would more than likely be low-end to keep costs down. The Alienware X51 started out at the low-end with an i3-2120 and a GTX 555. I'd seen it running (at Fry's) an older game like Jane's Advanced Strike Fighters and they had the resolution set to 720p. It wasn't a very good example of an affordable gaming box, with a price of over $700.
I would imagine that the Steambox would have to run on something like an AMD APU setup with a discrete videocard like the 7790. Still not a really great setup, but it would be in the price range of the Xbone and PS4.