Of course any < 7 day trip trying to take in northern Italy and Paris is going to be rushed, and Venice is definitely a must see - there's no place else on earth like Venice - but trying to do Firenze, Siena, Venice, not to mention Verona, in less than a week is also going to require a lot of running around and skipping many of the must-see spots in each of those cities. In any case, I don't think waikeekee sees his trip as an exhaustive introduction to any particular region. And 3 multi-hour train rides plus all the logistical hassle involved in changing hotels isn't going to be any less time consuming and will certainly be more fatiguing than one longer trip to Paris.
For the amount of time he has, I would limit myself to 2 or 3 days in a couple of cities, and doing Firenze and Paris is just as legitimate a trip as concentrating on 3 or 4 cities in northern Italy, especially with a long relaxing train ride between the two. But bear in mind that I love train trips through the European countryside, esp. the Alps - tho make sure you get a window seat in at least a 2nd class carriage.
Oh ya, waikeekee, if you're Asian looking, be careful in Italy that cab drivers and restaurants don't try to over charge you. That's more likely to happen in Rome and the south, but it's wise to keep an eye on the bill wherever you go in Italy. I'm afraid the gullible and uncomplaining Japanese have set a bad precedent.
In any case, when in Paris take the Metro. Firenze is a small town and you'll probably be able to walk where you want. Don't want to sound like you dad, but always buy a map. Both in Paris and Italy the fixed price meals are often a good bargain. Stand-up coffee at the bar can avoid most of the price that comes with taking a table - tho sitting at one of the famous left-bank cafes in Paris is a a great way to check out the local chicks.
