http://www.computerandvideogames.com/385376/valves-steam-box-early-specs-and-what-we-know-so-far/From what I hear it will be able to output video onto EIGHT TV's if you wanted, so you could have a separate monitor and controller for each player that wants to play; and the game won't be too stressed out.
It will have 1TB of space for you, between 4 and 8 gigabytes of DDR3 RAM, a quad-core 64-bit x86-based 32nm processor running at up to 3.2GHz (with 4MB of Level2 Cache), an integrated graphics card containing up to 384 programmable cores and then of course you have the four USB2.0 ports and so on. Apparently the motherboard will be virtually available along with other things I'm sure that help save space on the system.
Gabe even talked about desires to incorporate Gazetracking technology and other "futuretech" into the Piston, oh and the pistons colors will be fully customizable.
Systems with similar technology run about 1000$-1500$ but I really don't think Steam will sell this for more than 600$ or they know they'll have a relatively small market.
There's also word that Steam's working on a portable device called "LittleFoot".
The Steambox should be out by the end of this year for public purchase.