Basically, all of it. The "North Bay" is generally considered to be Napa, Sonoma, and Marin counties - the three counties north of SF across the Golden Gate. The wineries are primarily in Napa and Sonoma. Napa is better known and somewhat more popular, but Sonoma has lots to offer also. For Cabernet Sauvignon, Napa is the best choice. For Zinfadel, go with Sonoma. If you have more than a day, you can go to both. Distances aren't that far.