Fear ye not AMD/integrated Intel graphics chipset user, for Valve's Doug Lombardi has said that while the "first wave of prototypes" for Steam Machines favour Nvidia cards, it's not exclusive to them.
Valve has "worked closely together with all three" of these companies for the purpose of "optimizing their hardware for SteamOS," and so Valve aren't picking favourites for the Linux-based OS.
Thea series of Nvidia chipsets from the GTX 660 all the way up to the Titan. Valve wants a wide selection of Steam Machines to see how they operate.
"Last week, we posted some technical specs of our first wave of Steam Machine prototypes,” Lombardi told Maximum PC. “Although the graphics hardware that we've selected for the first wave of prototypes is a variety of Nvidia cards, that is not an indication that Steam Machines are Nvidia-only."
"In 2014, there will be Steam Machines commercially available with graphics hardware made by AMD, Nvidia, and Intel. Valve has worked closely together with all three of these companies on optimizing their hardware for SteamOS, and will continue to do so into the foreseeable future.”
Only a handful of Steam users have been accepted into the Steam Machine hardware beta. SteamOS is being released separately and for free by Valve at an undisclosed date, meaning it doesn't rely on Steam Machines.