Valve said that members of the public picked would receive a "high-performance prototype" designed for users who wanted "the most control possible" over their hardware.
Valve acknowledged that the majority of games in its library would not run natively on its equipment during the beta trial, but the rest will work seamlessly via in-home streaming.
The firm said that those taking part would receive the boxes this year and could make changes to the hardware and software, including the installation of another operating system.
The news comes two days after the company unveiled details of SteamOS.
Valve currently offers its Steam platform as software running on others' systems - Microsoft's Windows, Apple's Mac OS, Sony's PlayStation 3 and various Linux-based software. It acts as a marketplace and a way for gamers to play against each other and share titles.