This so called SteamMachine prototype is nothing more than your regular PC. They don't have anything solid at this phase but that's fine because Sony didn't have console ready 7 months ago. They said games were running on PS4 on E3 but I doubt it. SteamMachines are at least 6 months far from launch. When they launch I think we'll see even AMD APU based SteamMachines, something that's not possible atm thanks to non existent driver support under Linux.
AMD is experimenting with OpenGL 4.4 atm. They are trying to achieve something performance close to what can be done with low level API (Mantle). They are certain extensions introduced in latest version of OpenGL API which could help them. If you want to know more about this subject follow tweets of AMD's OpenGL guy,
Graham Sellers.
Streaming tech they are using is most likely they same nVidia used on Shield. That means 99 USD SteamMachines will most likely be based on nVidia tech.
This is a decent Shield overview, and it shows how good streaming tech is. Latency is quite low
Edit: SteamMachines need to be affordable to succeed. I'm sure GabeN knows this, he worked for Microsoft when they did the exact same thing with PC and Windows.