Hillary is unlikely to be President not just because she's a woman, but because people would be too concerned that Bill would really be making the decisions. I think she also appears insincere too often. If she and Obama keep feuding, the more Edwards looks like a viable candidate. This might give Edwards the edge he needs to get the Democratic nomination and possibly the Presidency, but that's uncertain.
What would really clinch the Presidency for the Democrats, IMO, is if Edwards would run as Vice President to Obama. (This is assuming that Edwards doesn't feel too burned from running as Kerry's VP to consider running second to another candidate again.) It couldn't be the other way around because it would look too much like Obama was second class to Edwards. If Edwards backed up Obama, they would get black votes for Obama, and whites on the edge who won't admit they don't want to vote for a black man will be more comfortable with Edwards on the ticket. Because they're both handsome men, they will get a significant amount of women votes as well. I know race and hormones aren't politically correct reasons to vote, but knowing human nature, I think people will vote that way anyway.