The only candidates that have a realistic chance are Dean and Clark, and I'm not even that sure about Clark. I don't think it will be much of a contest at all, really.
Among Primary voters, all the others are pretty much a joke.
It really doesn't matter that late primary polls still show a 3-4 person grouping at the top ... as the primaries go on, the Bob Kerreys, the Paul Simons and the Paul Tsongases drop out ... natural fact of nature.
The race will distill to a three-person race soon enough ... Dean, Clark, and one other (pick either Gephardt, Kerry, Lieberman or Edwards). Add in a few winner-take-all states ... add in the fact that super delegates almost always go with the person in the lead on delegate counts (if not the presumptive nominee, then the most likely one) ... add in the fact that many states (like Texas) split their allocation among primary and convention votes so that by the time the state convention rolls around, the delegates have fewer effective choices than they did in the primary. In 1988, I voted for Gary Hart ... but by the time the convention rolled around, myself and everyone else had a choice between Jackson, Gore, or Dukakis (I went as a Dukakis del.).
There will be a similar shakedown of options as events develop. Ignore the everpresent "what-if" scenarios about brokered conventions. Not gonna happen.