Robert Fertig

Review of Musei Vaticani-Dipertimento di...

These museums have Roman and Christian history as ...

These museums have Roman and Christian history as nauseum... And it's awesome! Reasonably priced, and the lines aren't so bad either! I recommend getting a ticket in advance and using the free Rick Steves audio guide for the museum and Sistine Chapel (which is within the museum).
It's big enough you could spend days and days in here, so if you're very interested in the church, Roman art, or Renaissance art you can instead buy a full day or more ticket and tour.

Also note: if you are thinking of doing Capitoline Museums, the Vatican Museums have similar content and it's even bigger and better.