So the SteamBox prototype specifications have been released, and all we can say at CCL is wow! From the announcement it didn't seem like the SteamBox would be a PC replacement but from the quoted specifications, the prototypes are almost certainly going to be more powerful than most gaming PCs - at least more powerful than my own.
The full specifcations as released by Valve are as follows:
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
From reading the news post it seems that it will effectively be a custom built mini gaming PC, but rather than being locked down like some of the bigger brands, Valve are touting the ability to swap and change parts as the user wants, and barring the custom enclosure the PC will use off the shelf parts, so you could even build your own!
Whilst it seems that the prototypes are just special editions, I believe the reason for the high specifications is due to Valve not knowing what games the beta testers will be playing whilst also being built to overcome any performance burdens the SteamOS may have initially, and if your gaming requirements are different and you play a lot of Indie style games, you could look to build something a bit less powerful.
A full rundown of the details can be seen in this link