We get a lot of questions about where to stay in Tuscany, especially in wine country. Here are a few suggestions of places we know and highly recommend:
Borgo Casa al Vento - the owner is a good friend and the property and wine are great. This is in Gaiole, so if you want a base closer to Siena than Florence and the wine country south of there (Montepulciano, Montalcino) it is a great choice - and you are still in the heart of Chianti Classico. Suites, rooms, and a villa are available. Pictures of the 2009 harvest here.
Fattoria Corzano e Paterno - fantastic family run winery and farm, they make exceptional sheep's milk cheese from their own herd, as well as excellent wine. Closer to Florence if you want to take in the sights of the city. A "real" agriturismo - this is a working operation! See video below for a little more insight.
Fattoria Rignana - one of the first places I fell in love with in Chianti. Just on the edge of Panzano, deep in the woods, the wine is amazing, as is the villa, with its own chapel and fantastic pool overlooking a valley full of vineyards, olive groves, and the hills beyond. Breakfast served outdoors in season. Great restaurant nearby. Pictures of the 2008 harvest here.
For all of the above I would definitely rent a car.