According to this article, Valve may sell a prebuilt miniPCs as game-console devices.

The rumor also states that the AlienWare X51 may have been designed to be the base (prototype) configuration, and the first version running win7-64bit would include an i7 Sandy Bridge CPU, 8GB RAM, 1TB HDD space, and a Nvidia GTX555 discrete GPU with changes possibly coming only in every three to four years.
Valve also filed a patent for a proprietary controller with user swappable components, but other USB devices would be also compatible of course.

While this might give a boost to PC gaming in general, it could also further slow down how PC technology advances imo. Valve will probably price it agressively low to gain market share against the other consoles, and if developpers design games with such system spec in mind, there will be not much (mainstream) demand for faster configurations until they release the next revision of the hardware.

Update: Apparently they were only using the X51 as a test configuration to develop their software, so the hardware production itself will be open-ended, and various manufacturers could build their own versions following some specification guidelines laid down by Valve, like the mandatory upgradeability with Valve's Steam Box firmware.
The device will also take advantage of Steam's "Big Picture" mode, and will introduce realtime biometric feedback via a bracelet integrated with the main controller. (I think Nintendo had something like this cancelled a few years ago)

The bottom line is that this looks like a very powerful plan to revolutionize the gaming market and to give some boost to the PC as a platform. They might come out with more info at the upcoming Game Developer Conference.

Update2:just like a simple mini pc:)