When to book?

There is no real answer to your question. You book it when the price is at a level that fits your budget. If you try for the cheapest ticket, you generally miss it because you were waiting for it to go cheaper. Start watching the prices now, use a fare tracker program, and decide what you can afford and then buy it. Four to six months is adequate for the other items.

I like to set up a daily rate notification on sidestep.com I watch the price for awhile to get an idea of what a good fare is likely to be, then buy when it seems like it's a good deal.

One other thought is that many airlines are cancelling booked flights these days. I haven't had the issue but some are saying that they refund your money and force you to buy a new ticket at higher prices rather than allowing you to change at the fare you already paid. Because of this, it may be a good idea not to purchase a ticket until four to six weeks before your trip. I'm not sure that's a great strategy, but it's something to consider.

Once you have determined your itinerary, you can book hotels. 3 months in advance is not to early to start booking. You can start doing research already: finding hotels in your price range, getting recommendations, etc. The good thing about a hotel reservation is that you can cancel if you find a better deal elsewhere.

There are some attractions that you need to book in advance, too. You can check them out and find out when you can book them - probably around 3 months in advance.

In addition to what others have said realize that into Venice and out of Rome can be quite more costly than other airports. At this time it is difficult to know what a cheap airfare will be. However I would usually project to get your fare by feb-Mar. and ck with all consolidators online for a guide of fares. Orbitz, cheaptickets, expedia... In 2008 june-july from Sfo was about $1600 &up