Valve has revealed the specs for the Steam Machine Prototypes.
The machines will have the following:
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