Obviously, McCain and Romney face the highest stakes tonight. If McCain fails to win in NH, his candidacy will be in very deep, probably irrecoverable trouble (he's actually on the brink of running out of cash now). A narrow loss to McCain would be another hard blow for Romney, but he might still have something of a chance to win the Michigan primary one week from now and get back in the race. A non-narrow loss to McCain, and, well, the Romney candidacy will continue on (he has the personal wealth to continue all the way to the convention) but without much of a point.
The contest for third will be well worth watching as Huckabee, Giuliani, and (yes) Paul compete to exceed expectations. Don't be at all surprised if Huckabee takes a strong third (not a prediction, just saying).