if America fell today, I could only imagine the shock and chaos that would bring about. all the money, all the land, everything would be total horror. when the mighty countries of old fell look what happened, chaos and they were swallowed up in other countries and then their people because that people. this will also happen to America, auctioned off perhaps? sold for what was own? what of the people? what of the true natives who are full blooded and have always been here? they would probably be sucked back into another culture, will they be allowed to remain? will the American people become slaves? so many questions which could arise if this actually happened today or any day. it would be a horrible event which could bring about even more chaos. countries fighting over the remains of America, it's scary if one really sits down to think about it. history alone shows what happens when a mighty empire falls, this would probably not be any different.
I know the US has caused some major problems, but the world would not be better off if the US collapsed given how much of a market that the USA represents. I think a better scenario would be one where the US gradually declines while other nations rise up and a level of equilibrium is established throughout most of the world.
There is no chance the US will collapse, however it's just a matter of time until another enemy of the US decide that they have had enough, and retaliate, whether America has stealth jets, aircraft carriers or nuclear warheads.
As things stand in the world, the world probably wouldn't be a better place with the fall of the US, it would certainlly be different and more interesting. Brazil and china would rise as new superpowers to fill the vacuum, human rights as we know them in the west would start to erode and eventually we'd all end up with about the same level of rights they have in china, or we'd have large scale wars ravaging the world again as a new equilibrium tries to establish itself.
I don't think Brasil or China would rise as new superpowers quite yet. While they definitely are powerful economically, they don't have broad international support, and they don't have the naval power to rival the existing blue-waters. In time however, I'd bet on Brasil, if only because of a more progressive Government and a head-start.