i would put Rome 3rd.
Florence has 2x the Rennaisance art that Rome has, and Siena is just kick-ass. Venice you visit just to visit...b/c you have no choice in the matter.
Avoid Milano like the plague, or just go straight to Como if you're in the area. (you have no choice but to pass through if taking a train/bus from Switzerland so...)
IF it's soon, then you won't be in peak tourist time, so a little easier, but still cold (Florence is about the same latitude as VA, so they get similar weather) Tuscany is all about hills and valleys, which means that a lot of the spots you'll be walking around can get chilly.
Try to spend time here:
1) Florence (1 week is minimal)
2) Venice (~4 days is good)
3) Seina (can be part of Florence trip, spend a day there)
3a) various villages in Tuscany (San Gimignano, Greve, Panzano, Assisi if you can get there (which is in Umbria I believe)...whatever)
3b) La Cinque Terra. If you like to hike and love incredible views in quaint towns with great food, these isolated cliff villages are spectacular. Heavy during tourist season. Very little english spoken, though. actually takes ~3 days to see it all (relaxed). The hike is easy if you've hiked moderately only once in your life
4) Rome. ...it's Rome. why not? you must see the Vatican. check out the Forum, and the tomb of Julius the second is very close to the Colloseo/Furo area (Michalengelo's "Moses" is there; largely considered his greatest work Others: David--Florence; La Pieta 1, Rome. La Pieta 3, Florence (my favorite), Last Judgement/Ceiling, Rome.
(can be done in 3 days)
IF you're an art whore, spend your time in Florence. Rome has great stuff of course, but simply can not compete with what Florence has to offer. (Uffizi, L' Accadamia, Museo del' Duomo) Both have excellent cathedrals and churches to check out. IF you need some more suggestions for Florence, then let me know....