"The Steam universe is expanding in 2014."
The new GTX 8** series will probably be out by the time Valve's Steam Machine is coming out. It would make sense for them to use the newer cards.
Also, the prices won't be as high as everyone is saying ($2000). Since neither console used nvidia this time, nvidia is almost certainly giving Valve a price cut to use their hardware. That's why nvidia has been saying things like
"Consoles will never be on par with PCs" (completely true, but unusual for a hardware company to come out and say that)
"We're making open source drivers"
"We're working with Valve for SteamOS"
And also literally every prototype has nvidia cards. I know I just said the hardware beta isn't finalized but I was referring to the series; the final hardware will almost definitely be nvidia.