While I recognize it's a matter of personal preference--and I do like Siena a great deal, too--I have to disagree with Scott. If you can, try to spend at least one night in Firenze, which I prefer to Rome, while admitting one has to do the Vatican and the Coliseum and the night-time walks through Rome. Florence is stunning. Apart from the museums with such breath-taking art, the Arno is beautiful (and the neighborhood south of the Arno, with Piazzele Michelangelo for its remarkable views and great restaurants in a more residential setting... in Rome the Tiber is a bit of an afterthought), Brunelleschi's dome, a miracle of art AND engineering, the Santa Croce area, Ponte Vecchio, and Piazza Signoria... Having just visited Rome and Florence with my wife this past winter, three nights in Rome is plenty, for my money: I'd add a few nights in Florence. Florence has a geography and a center and a plan one can get his/her mind around, grasp relatively quickly, and yet it's dense and rich with culture and sites and people and food and music. Florence is so much more than a day trip with a few museums to check off your list. Why not stay a while?