FYI - If you're planning to come to Rome the weekend of 11/21 - 11/22, the Sistine Chapel is closed both days. Sunday is the normal "closed" date, but on Saturday the Pope is welcoming artists from around the world. See this story by By Sarah Delaney of the Catholic News Service for more info: Pope to meet artists in Sistine Chapel to rekindle faith-art dialogue.
You may need to readjust your plans if you're coming to Rome this week. Do know that Friday, 11/20 and Monday, 11/23 will be MORE crowded days as a result of this Saturday closure. If you are locked into this week, you may need to purchase "skip the line" tickets at the Vatican Ticket Office. CLICK HERE for the Musuems Tickets and Info.