We visited both Siena and SG the same day when we were in Italy two summers ago. SG is a very charming place with an authentic feel to it, but one day was enough. Siena is more striking, and larger.

We thought about attending Palio when we were in the area, but all the locals warned against it for a number of reasons. You'll probably have to stand all day amidst hundreds of thousands of people, and the campo holds the heat very well. That being said, it's supposed to be great fun to be there in the days leading up to the race, particularly at the cafes at night.

La Cisterna is our favorite hotel in S.G., beautiful views, perfect location and reasonable price. Google for their website.

I'm planning a very similar trip and I we are probably going from Rome to San Margherita (sp?) for one or two nights, so we can see Cinque Terre both by boat and by hiking. Then we go to Florence and will see San Gimignana (sp?) on one day-trip from there. From Florence we go on to Venice. That way you could add more time to Venice, too. Four nights between Siena or San Gimignana look like a long time IMHO.