Steambox is supposed to be hardware independent as such. It can use AMD, Intel, nVidia. Essentially its just a tiny x86 PC. Valve also said they wanted the steambox to be upgradeable. I assume you keep the chassis, PSU and simply upgrade x amount of the internals.
However if they keep that silly Piston design, rather than going mITX. Its not only gonna be expensive, but also rather limited.
And 999$ for an AMD
R464 with 8GB memory and 128GB SSD is not exactly cheap or a good deal in any way.
The big platform for Steam is the SteamOS. Anything eslse is just accessories. And nobody needs a Steambox if you already got a PC. Just install SteamOS on the PC.