Valve reveals specs of Steam Machine prototype

In a post of the Steam blog, Valve said that the prototype machine is a "high-end, high-performance box, built out of off-the-shelf PC parts". It is also fully upgradable, allowing any user to swap out the graphics processing unit (GPU), hard drive, central processing unit (CPU), or even the motherboard if they want to.
The Steam Machine measures 12 x 12.4 x 2.9 inches and will ship with a mixture of GPUs – some with NVidia Titan, some GTX780, some GTX760, and some GTX660. It will also have a mixture of CPUs – some boxes with Intel i7-4770, some i5-4570, and some i3.
The console will have 16GB of RAM for the CPU and 3GB for the GPU. It will also have 1TB/8GB of hybrid SSHD storage, and an internal 450w 80Plus Gold power supply.
Valve will ship 300 of its prototype consoles to Steam users. However, the Steam Machines available for sale next year will be made by a variety of companies – some of which will be capable of meeting the demands of lots of Steam users very quickly, while others will be more specialised and lower volume.
As a result, the hardware specs of each of those machines will differ – in many cases substantially – from Valve's prototype.