The Gabe Newell interview seemed to suggest they were going with a pretty top end GPU, because he was discussing the new Nvidia architecture that's coming, and how it could allow you to use a Steambox as a game server, offering up different games to different people in your household. I don't know. Gabe can talk a lot of shit sometimes.
"The Steam Box will also be a server. Any PC can serve multiple monitors, so over time, the next-generation (post-Kepler) you can have one GPU that’s serving up eight simultaeneous game calls. So you could have one PC and eight televisions and eight controllers and everybody getting great performance out of it. We’re used to having one monitor, or two monitors — now we’re saying let's expand that a little bit." -
http://www.theverge.com/2013/1/8/3852144/gabe-newell-interview-steam-box-future-of-gaming