Why not just go with the big GPU anyway. Granted the console will technically be more powerful than a PC built with the same specs because of the lack of overhead; but why not just go bigger. If they went with a 7870 or 7950, the could build the system to use the on-die 6550D equivalent and only turn on the big GPU when a game is loaded. It would use less than 1W to keep on standby so WTF not? It would also eliminate potential crossfire issues and would have more power overall. Hell the could use the on-die GPU as a physics processor while gaming using Direct Compute or AMD APP whatever.
I think both Sony and AMD are missing an opportunity here or there is more to come.