The Valve console namely Steam Machines (as they'll come in different shapes and sizes) now has some preliminary specs for the few hundred prototypes that are being made right now.
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