I guess all they have done is look at what can deliver solid 1080p gaming at around 30 minimum-60fps maximum.
I'm pretty sure a modded/custom/hybrid 6670/6770 with an optimised DX11/12 API, custom Cell style CPU and some tweaky cache/ram could well do that. With all those optimisations no reason why a 6670 couldn't perform as well as a 6850 or better I reckon.
1080p is the bar now. Do you need a 6970/580+ for that? No. Its whatever will do the job for the minimum cost per box. I don't think the next gen of consoles will be such a big leap as the current gen were. More evolutionary than revolutionary. That means cheaper parts.
Sony and MS will be wanting to get some profit back on each box far sooner than they were back in 2005/6. Shareholders and the market cant sustain such losses anymore.
man i thought the general consensus after a few pages of discussion was that this was fud and not very likely. if it really does end up being a 6670 theres no way in hell id waste my money on it. 6950 or 6870 would be great i think if they want it to last 8 years again and not suck balls.