I think Evan Bayh can win the nomination, he will be a formable opponent.
Bayh really has no chance at winning the nomination barring some major geopolitical event. He's been trying to move left over the past year+, but that will do little for him as progressives won't pass up Feingold or Gore for Evan Bayh, regardless to his vote on John Roberts. Warner on the other hand has some semblance of a chance; he comes off as bright and doesn't have a long "hawkish" voting record, rather just a crisp term of executive experience. But still I think the primary will come down to the establishment (Hillary Clinton) v. the antiwar left (Al Gore if he runs or Russ Feingold).
As for Webb, it's cool that he's making a run, but I really don't think he has much of a chance. Virginia however is looking to be more winnable than Tennessee, for what it's worth. Even if he secured more money I don't see Webb's chances being any better than one in five. As for Allen in 2008, he's absolutely finished before he started and may not even run.