A Titan! As in, yes, an Nvidia Titan!
Later this year, Valve will ship out 300 prototype Steam Machines for beta testers. If you thought you wanted in on this action, just wait until you see the official system specs. "The prototype machine is a high-end, high-performance box, built out of off-the-shelf PC parts," says Valve. "It is also fully upgradable, allowing any user to swap out the GPU, hard drive, CPU, even the motherboard if you really want to."
For reference, the current slim Xbox 360 4GB console's dimensions are 12.4 x 11.9 x 7.4.
As we know, multiple manufacturers will be introducing their own SteamOS-powered systems next year. "Some of those companies will be capable of meeting the demands of lots of Steam users very quickly, some will be more specialized and lower volume. The hardware specs of each of those machines will differ, in many cases substantially, from our prototype."