Ardenas Barehand:
... I'm still a little unclear how Clerical Investment, Power Investment, and plain old Magery interact.
Clerical Investment is one part Social Rank, one part Divine Rank. It is not required for Holy powers or skills, however, it is required to perform Proper Rites (in DFRPG it's necessary to Lead Prayers and Bless Altars). It's also good for a 10% reduction in price on certain Holy gear and better prices when selling to temples.
Power Investiture is basically Divine Magery, except it doesn't come with Magery 0 abilities and isn't beholden to Mana, instead relying on Sanctity. Otherwise, Holy spells function identically to Magic based spells and interact with another seamlessly.