Well GT5 is reported to be 1280X1080p. The only real 1080p that was demonstrated was GTHD, which was of course canceled.
Anyway, with all the rumors flying around, I am personally preparing myself and saving up for the PS4. I don't why, but the GT in me needs to have GT6 as soon as possible, and it's starting to get blatantly obvious that it may very well be a PS4 game. It doesn't hurt to save anyway.
Other than that, I personally think GT6 can easily be a PS3 game and still be a big hit, but the clash between the game and next generation makes it very unlikely. But seriously, if we want weather/time change in every track, we can. If they can do it with the Nurbrugring 24hr, why not other, smaller tracks. A-Spec certainly doesn't need next gen for an overhaul. Cars already look stunning, including the interiors. 12-16 cars per track is good enough for me, and even if it drops, the framerate is pleasing. I'm also satisfied enough with the physics, for this gen at least. Bottom line, like GT2/GT4, if GT6 were for the PS3, I believe it would definitely be just fine, and not like how some people portray it like it's an absolute dumb decision.
Of course, next gen will bring more potential. I mean that's the point. Although I want GT6 to be for the PS3, I can't deny that it will probably be many times better if it were made for the PS4. You can't argue against better visuals and physics as those two alone are a big part to the experience that's worthy of admission. We just have to see it for ourselves. Hopefully, as soon as this year's E3.