I'm Not too sure.
Here is the specs I found for the 300 prototypes they are mailing for testing. (I don't know if these are correct now, It was just a basic google search)
The 300 prototype units will ship with the following components:
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
overall it sounds pretty awesome. I don't know the pricing yet since there are going to be multiple steam boxes.