A late addition... but not too late since you're going in July: http://www.fodors.com/community/euro...ightseeing.cfm

I don't think anyone has mentioned the view from the top of Castel Sant'Angelo. It has beautiful views of Rome, the Tiber and the Vatican. I also like the views from the highest level of the Monument to Victor Emanuel -- Piazza Venezia from the front and the Forum from the back.

I agree with the suggestion of San Luigi dei Francesi. I would also recommend Santa Maria del Popolo; it too has wonderful Caravaggios. If you were to walk from the Galleria Borghese through the gardens to the Pincio park, you would have the views of Piazza Popolo and can then go down the stairs to Santa Maria.