From the Steam Universe group thread. Includes specs
http://steamcommunity.com/groups/steamuniverse#announcements
This is just a prototype for 300 users to test the device with SteamOS, and in no way a prediction of what's to come for all SteamMachines buyers.
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 DDR5 (GPU)
Storage: 1TB/8GB Hybrid SSHD
Power Supply: Internal 450w 80Plus Gold
Dimensions: approx. 12 x 12.4 x 2.9 in high
http://steamcommunity.com/groups/steamuniverse#announcements
It is also fully upgradable, allowing any user to swap out the GPU, hard drive, CPU, even the motherboard if you really want to.
And to be clear, this design is not meant to serve the needs of all of the tens of millions of Steam users. It may, however, be the kind of machine that a significant percentage of Steam users would actually want to purchase - those who want plenty of performance in a high-end living room package. Many others would opt for machines that have been more carefully designed to cost less, or to be tiny, or super quiet, and there will be Steam Machines that fit those descriptions.
This is just a prototype for 300 users to test the device with SteamOS, and in no way a prediction of what's to come for all SteamMachines buyers.