One other little word of advice. Obviously you can buy souvenirs where ever you want, but some things that I found were as follows:
1.) In Rome, if you want to buy clothing, this would be the place for it. When I mean clothing, it's probably the same stuff that you can buy in the states, but it would be from Rome.
2.) Vatican City...Obviously, anything religious you would want to buy here. I bought rosaries that were blessed by the Pope. They vary in price from anywhere between 12 Euros up to 100 Euros.
3.) Orvieto...if you want to buy some very good wine at a very low price, you will have time to get it here. You can buy it here for about 3 or 4 Euros a bottle and it's very good. You can also get Pottery here. They have beautiful pottery here.
4.) Tuscany...more wine here. I didn't buy any wine here, but it was also very good as it's made in Aritimino. I think it was a little more expensive than in Orvieto, but certainly not expensive. The one thing that I would highly recommend to buy here is Olive Oil and Balsamic Vinegar. It is by far the best that I have ever had and as mentioned above, I also wish I had bought more.
5.) Florence - Leather, leather, leather. If you ever wanted that perfect leather jacket that looks oh so good on you....buy it here. It is really good quality and reasonably priced. I wouldn't call it cheap, but not outrageous either. They sell scarves here too which are really cheap. My wife was a little disappointed that she didn't get any since you could talk them down to about 3 Euro.
6.) Venice - Masks (you can get masks of all sizes here). We bought a whole bunch of little ones for 3.50 Euro eachas Christmas presents. They can be used as refrigerator magnets or Christmas ornaments. We could have bought them cheaper in Florence, but I personally think they were nicer in Venice and we wanted to buy Venetian masks in Venice. My wife also got a Murano glass necklace and earrings in Venice. They were pretty reasonable too, but you'll definitely have to shop around for good prices.
If I was to do it again, I would have packed some bubble wrap in my carry on to wrap more olive oil up and bring home...it was that good. The place in Orvieto recommended that she wrap our pottery in bubble wrap and take it with us rather than shipping it due to the cost of shipping. Some places will ship it to the US free of charge, but I'm guessing their prices are a bit higher to cover the shipping. Believe me, there are many other things that you can purchase in all of these places and it really depends on your taste and style. I just wanted to give everyone a little bit of my personal advice because you always end up wondering "If I buy wine here, will it be cheaper somewhere else?" or "If I buy these Venetian masks in Florence, could I get a better price in Venice?". Hopefully this helps.