I think you're setting yourself up to be sorely disappointed. PCs will be lucky to be 2-3x as fast by the time the next consoles launch. Those are 500-600w beasts. With a third that power as a ceiling, I'd say the next gen consoles are going to be hard pressed to hit 20x PS360 levels, more likely 10-15x or so. Furthermore, reguardless of your like or dislike of diminishing returns, its a real thing, and the closer you get to realism, the harder it is to tell the difference between power levels. Many polygons spend their time below the size of a pixel now, whats more polys gonna add? How much smoother is a 200k poly model compared to a 20k model with todays techniques? Not a lot.
Still, lighting, shading and IQ are far away from offline CG level. There is plenty of room to improve the graphics, but it's going to be expensive and it will require time.
I hope that the next Xbox will use AMD Graphic Core Next.