U.S Vs China right?
So not:
All of Africa, (perhaps not Egypt, Morocco, South Africa, Congo)
South America, perhaps a bit of Mexico would be hit, Greenland, Australia/New Zealand, Sweden, Norway, Finland, Portugal.
I don't think that most of the world would be hit. Then again, if China and India got hit then it's 1/3 of the world would be dead.... except for all the people who survive

.
You have to realize that if the US and China nuked each other they would focus mainly on each other, and other nuclear powers, then their allies. So it wouldn't really be a world apocalypse.