@Ace: I'm atheistic but I did have to go through Catholic teachings from my family, so I'll answer your question from the Catholic point of view.
How monotheistic is christianity? God is apparently three beings and people also worship and pray to saints, to Mary, to angels.
According to the Catholics, God has 3 distinct
forms. God is all-powerful, so existing as 3 different entities is not exactly a hard thing for him to do. You have God the creator, God the son of man (Jesus), and God the spirit (Holy Spirit). It's all the same God, however, different forms with different purposes.
Angels, Mary, and the Saints are
not worshiped as Gods. They are not all-powerful. If you believe that we all have a spirit that goes to Heaven, then calling upon these people with prayers is like asking guidance from a specific person who's knowledgeable in a specific field. You can pray to anyone, even your deceased loved ones. However, you wouldn't be worshiping them as all-powerful, all-knowledgeable beings which created everything.