As everyone already said, there will be crowds, but there are always crowds in Rome no matter what time of year. The Rick Steves Rome guide book is an excellent source to have. I would look at the chapter on Rome churches and see the churches as that is where the great art is in Rome. The churches are so huge that you won't feel crowded and they are amazing. And they are free!
But remember you must not wear sleeveless tops, low cut tops or shorts in the churches. Also go to an outdoor cafe on Piazza Navona, very crowded piazza, but have a drink at a cafe and watch the people. You will be sitting down at your own table. Make a reservation as soon as possible ( don't know how far in advance but check the website) for the Borghese Gallery as suggested. They only let so many people in for a 2 hour visit. You won't feel crowded. Have a gelato at a cafe, any where you can sit down for a short rest, will make you feel better.
If you don't like crowds go to the Doria Pamphij Gallery, ( in RS Rome book), I went two years ago and it was a nice museum with an english audio guide and hardly anyone in it. Not many people go see this as everyone wants to see the major sights. It is off the beaten path. Have a great time. Eat gelato, drink water, wear very light clothing, sneakers, and sunscreen. Have a safe trip.