The Uffizi gallery is one of the great museums of the world. The building was created by the Medici family in the 1560s as a commercial building and the top floor was designed to house their great art collection. Situated right on the Arno, the building, designed by Vasari, is a work of art in itself. The gallery is designed and laid out to give you a chronological tour of Italian art from midievel times through the late renaissance. I suggest you get there 30 minutes before they open so that you get a chance to view this marvelous collection with the least amount of people possible. The cost is 14,000 lire and will be the best money you spend while in Florence. There are several must see items in the collection. First, the Botticelli room has the best collection of the man's work in the world. His masterpiece Primavera is here as is the Birth of Venus. Up until a hundred years ago Botticelli's work was not appreciated. As a matter of fact, most people went to the Uffizi and ignored the painting to view the sculptures in the hallway (which are wonderful as well).
Other highlights include a few masterpieces by Raphael, "The Holy Family" by Michelangelo and a couple of beautiful paintings by Leonardo
Di Vinci. Make sure you look up in the corridor! The frescos that adorn the ceilings are fabulous.
There is a website for the gallery that you might want to check out: Uffizi Website.