Clinton and Obama are simply head and shoulders above the rest and the shoot-out will between the pair of them. Edwards is well-respected but gradually losing headway.

I understand how you could get this impression from the US media, but the truth is somewhat more complex.  First, national polls have no meaning, because the first actual election comes in the Iowa Caucuses where the polling is entirely different with Edwards far in the lead, followed by Clinton, with Obama and Richardson (this being something recent) fighting for third place.

The Iowa caucuses are notoriously unpredictable, because in order to get a polling stations votes, you have to reach viability, which means that if you're candidate isn't above a certain threshold, you have to vote in what amounts to a second round.  It's vaguely runoff electionish.  Edwards has a tremendous group of supporters in Iowa that have been in place since 2004, and has consistently led in polls of that state.

Once a candidate wins Iowa, they have on average seen a 14% jump in the polls.  So for example in 2004 John Kerry saw his polling in the next state to vote, New Hampshire, saw his poll numbers rise by almost a third, while the then frontleader in national polls, Howard Dean crashed.  

Iowa is the only state where polls have any real meaning at this point.  Giving the shift in the election calendar pushing many states to Feb 5 or earlier (Iowa will vote in early January.), the results of the Iowa caucuses will be even more important than before.  And national polls will have even less meaning.

The media though, doesn't seem to have gotten that particular memo.

And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg