Will America fall? Yes.  Anytime soon? Most likely not.  Every nation has a beginning and and end; even the Romans eventually fell to the barbarians.  The United States has faced and overcome much greater adversity than it is facing right now.  The economy is undoubtedly slowing down, but its not the first time.  We have had much harder times economically and come out of them to prosper again.  Oil prices are rising, sure, but they won't be at $200 per barrel for a while.  During the 1970s oil crisis, some experts predicted prices in the hundreds of dollars within the decade, but that never happened.  The slowing economy we are facing now is the result of the surging one we had a few years ago.  It is a return to equilibrium.  The US, or any nation for that matter, cannot sustain large growth for long periods of time.  We are just on the downward slope of the peaks and valleys every economy experiences.  

In regards to American currency losing value, this is both a factor of a slowing US economy and growing economies elsewhere.  The EU is an economic powerhouse, so their currency should be valuable.  The Canadian dollar has also fallen to almost exactly 1 US dollar over the last month or so, so that exchange rate is nothing to get too terribly excited about.  The NAFTA superhighway that you mentioned is much more an idea than reality.  If you honestly believe that the breadbasket of the country is going to allow our government to build a highway through it that will cripple its economy by allowing cheaper Mexican food imports, then you don't know much about the midwest.  

Remember, this is still a government for the people and by the people.  Yes, the current administration has taken some liberties with the constitution and done things that a majority does not agree with.  But, the beauty of the system is that the American people can put leaders in place that they believe will point the country in the right direction.  The Bush administration has not done irreparable damage, we can only pray that whoever inherits the reigns will be able to keep America in the successful direction its been headed in for the better part of the last century.