I've never been on a formal wine tour, (we usually just visit a few winteries take the public tour and taste) but we live close enough to go up there once or twice a year.
Generally, if you want to taste all day and go a few different places, and everyone in your party wants to taste, I would reccomend taking either a group tour or renting a limo, so that you don't have to worry about getting too tipsy to drive. A bus tour may have an arranged set of places to visit and have your tasting fees included in the price, whereas a limo will take you wherever you want, but you'd have to pay extra to taste.
As for tasting, in Napa, and to a lesser extent in Sonoma, you generally will have to pay a tasting fee to taste, anywhere from $5-20 depending on how much the winery thinks they can get away with charging. Usually this is for 3-6 tastes of wine and will often be waived (or taken off your purchase) if you buy a bottle or case of wine.
In addition, some wineries offer free or paid tours of their facility, showing how the wine is made. The paid ones are usually better than the free, but after you've been on one tour for still wine and one for sparkling, you probably never need to go again. Usually you can skip the tour, walk right into the tasting room, plop down your $5 and taste.
Some of the fancier places (usually the french owned Sparkling wine producers) have very nice, and very expensive tasting rooms which are table service and are really aimed at selling you food as well as wine.
There are many wineries (over 200 between Napa and Sonoma) so I'd do a little research before to figure out what wineries I'd like to go to (I've only been to about 15 in the 8 years we've lived here) or call a tour company and rely on their reccomendations.
let me know if you have more questions or if I've confused you further.
I don't know where you're staying in Napa, but I personally enjoy going to some of the places near the town of Sonoma (just over the hill on rte 121) and they tend to be cheaper to taste and wine that I like and can afford to buy.