As I said earlier, start at the end of Napa Valley and work your way backwards. If you plan on visiting a lot of vineyards during the day, you'll thank me for this. On your way to "the end" stop at Dean & Deluca for some munchies (I suggest some of their bread). If you want to see a natural sight, head down to Yellow Stone geyser were you can get a AAA discount (at least when I went).
Now, when you start your trip, start at Sterling Vineyard. Its roughly $15 for the tour and tastings. This includes a gondola ride up the mountain with breath taking views. Its a self guided tour with some video clips. You can purchase the wine at the top or bottom of the mountain. Very friendly staff!
Next, head over to Larkmeade Lane to visit the Frank Family Vineyard. It is sort of off the beaten path but it is so well worth the trip. (This is where the bread will come in handy). The tastings are free. You start of in the main room where you can buy the wine/champagne and taste three different champagnes. Next you move onto the next room where you taste several of the wines. The Zinfandel is my favorite, they call it the L.P.R. I'll leave that mystery to you!
Head over to Beringer for their tour. If you are claustrophobic, skip the tour. The tour consists of caves where the barrels are kept and the barrels may even be there so it is even more close together. At the end of the tour you get to taste a few of the wines. Although this tour is cheap, it is ok but you can skip it. If you like Beringer, just go purchase the wines and skip the tour. The grounds are pretty and a great place to have a picnic lunch.
My next recommendation is Robert Mondavi. If you go in the winter, the tour is shorter. However, if you go in the summer, you get to see the actual vineyards and how they label the different rows of grapes. You get up-close and personal with the large wine barrels where the grapes ferment. Then you go through several rooms where there are just barrels and barrels of wine that you can touch. The tour ends with a wine tasting, where one of the glasses is paired with food. The recipes are available for you to take with them.
Although I did not go to Sutter Home, I have heard that it is just a shop and no tour. Like all the wineries, the bottles are considerably cheaper at the vineyard than at your local liquor store. Some places will offer you free shipping or other perks if you purchase a case. If you don't want to purchase a case, have the vineyard package it for you the best (and cheapest) they can to take back with you. Some airlines now are putting restrictions on weight, so pack a duffel bag in your luggage just for your wine.
On your way home, after you of course have "eaten your bread," check out the outlets on your way. Although it is a little on the smaller side, they do have good shops with some good bargains.
You can do Napa Valley in a day, but make sure you take turns and monitor your drinking, because there are so many vineyards and so much wine to try. Some other vineyards that are there but I haven't tried are St. Supery, Whitehall Lane, and Beaulieau Vineyards. Good luck, and happy drinking!