Had you asked me two months ago, I would have said Dodd, Richardson and Biden are the only viable "Tier Two" Democratic contenders. (Clinton, Edwards and Obama being Tier One.) Having seen a few of the debates, I think Biden and Richardson would be flayed alive by any of the "Tier One" Republican candidates (Giuliani, McCain and Romney). Biden just doesn't know how to take a punch, and Richardson (who is still my favorite candidate) just gets flustered too easily and ends up saying dumb things (like being homosexual is a choice). Dodd just has little charisma on a national stage.
If a Tier Two GOP (Huckabee, Paul, Thompson) somehow wins (they won't) I think Biden, Dodd or Richardson could take them on. It would be an even fight. I don't think either of the Tier Three Dems (Kucinich, Gravel) could take any of the GOP nominees (even Tancredo, the Tier Three candidate).