The only GOP guy I'm seriously worried about is Lincoln Chaffee. Possibly McCain, but he might be more media-hog than serious threat to switch.
It comes down to that. If we get 53 seats, we'll be in good shape, and that takes us to 2004, where we knock off four or five more Dems (Giuliani vs. Schumer, somebody vs. Daschle, we nearly got Feingold in `98, and Reid in NV, maybe Boxer in CA if we can get Bill Jones to run).
As I said, we have to remember the lesson of the parable of the old bull and the young bull. If we do that, we will win. If we don't, we lose.