It really depends on who's nuking whom and what the state of the world is at the time.
If Pakistand nukes India, neither side has enough firepower to hurt the rest of us, and only China would be have a real interest in the outcome.
If Iran nukes Jerusalem, things could get entertaining, but again, I think the major players, the US, Russia, and China, would try to stay out of it.
If a random towelhead nukes a US city, the result should be the decisive and immediate destruction of Mecca, Medina, Ryadh, Tehran, and any other handy place on our "enemies" list.
I doubt seriously if Russia has futher nuclear ambitions against the United States. China keeps saying stupid things, but for now they lack the means to back their threats up. Clearly they lose the most by any nuclear attack on the US.
What we need to do, of course, is develop and build a strong missile defense.