Silverado Trail is *much* better than Hwy 29 for bikes, as a north/south route. But even that is pretty darn busy, to the point of being unpleasant- try to do your biking in the mornings on weekdays if at all possible. Hwy 29 is downright scary, especially on the weekends- bumper-to-bumper cars with people who mostly have been drinking, IIRC. But since most of the wineries are on Hwy 29, you would need to take some of the smaller east-west routes to get to them. Any winery that you go to is nice- I liked V. Sattui for the cheese and bread selection, and congenial picnic grounds. I enjoyed staying in Calistoga- a massage and mud-bath at Doctor Wilkerson's spa was a good interlude, and they have relatively inexpensive motel rooms there too. I never had a bad meal in the Napa Valley, but I didn't eat in any fast-food restaurants either.
Good luck and have the best of times!