There is no single "Steambox" as far as I can tell, there will be new models every year, much like the iPhone or tablet model.
Perhaps Steam will even allow simple hardware upgrades too. A console, once you get it, that's basically all she wrote for the next 6 years.
No body will upgrade a Steam Box and the parts are custom... so even more expensive than PC parts.
Semms like the guys have no ideia whar the Steam Box is.