In their recent announcement about Steam Machines, Valve had stated that they will be shipping 300 units of their own prototype device to people who qualified for the beta testing of the units. Today they have released the details on the device's specifications.
They state that they wanted to build something special, something high-end, high-performance, built out of off-the-shelf parts to ensure that it was truly upgradable so users could pop in replacement parts whenever they wanted to do so.
The specs they released on the prototype are;
GPU: some units with NVidia Titan, some GTX780, some GTX760, and some GTX660
CPU: some boxes with Intel i7-4770, some i5-4570, and some i3
RAM: 16GB DDR3-1600 (CPU), 3GB GDDR5 (GPU)
Storage: 1TB/8GB Hybrid SSHD
Power Supply: Internal 450w 80Plus Gold
Dimensions: approx. 12 x 12.4 x 2.9 in high
They acknowledge that things will change over time, so they wanted to make sure that things were left up to the user's discretion. They are not quite ready to release a picture of the prototype units just yet, but we should be seeing some in the near future.