We'll have buildings 100 Km tall (AKA the boundary between the Earth's atmosphere and outer space. Search "Kármán line".)

Those pictures in the OP are beautiful ;_;

Hmmmm should I play Xenogears?

100 Km? That's a bit too much (and by a bit I mean a lot)

OT. I hope mankind will go 1 step back and try to mix cities with nature. More parks and forests, maybe with artificial rivers (like the Citadel from Mass Effect)

300-500 years is a long way from now and I think that most pictures are les than 200 years from now. I think humans will mostly live in giant cities with buildings that are selfsufficient in energy and water/air purification.

One thing I'm thinking though is maybe we'll find a way to change how we view transportation. I mean, we'll fake travelling physically and have some sort of mind travel using a device that makes you think you were there physically. I hope we don't get there but I think this is where we're heading.

There really isnt any way to predict how it will look like. So many things might happen that will change and shape the course of humanity. But I hope we get those cities like the pictures of the OP, except with a lillte bit more nature.

Ever see the movie Idiocracy, I'd imagine it would be like that.

http://www.youtube.com/watch?v=L0yQunhOaU0

there will still be no global warming.
and the earth will still be round sphere.
dang, i got beat to that joke

Like your first and last picture.