On Spak's suggestion, maybe this can be the official thread for the Steam Box. Here is what we know about Xi3's version of Steam Box:
It's launching Holiday 2013.
There will be 3 configurations. One will be $999 and come with a 128GB SSD. The next will be $1,340 with a 256GB SSD. The final will be $1,750 with a 512GB SSD.
All three will come with 8GB of RAM and a 3.2 GHz Quad Core AMD A10 processor. Valve has said they have nothing to do with Xi3's version of Steam Box, which is codenamed "Piston".