Questions about the Da Vinci Code

I just finished reading the Da Vinci Code. It's fascinating book. But what bothers me is that I don't really know how much of that is real and how much is not.

So my questions is, how much of the following is based on fact (or at least widely accepted by scholars today), and how much of it is flimsy and unsubstianted speculation

1) The existence of the Priory of Sion and the list of its "Grand Masters"
2) The symbology of the Rose, the Pentagon, the secret Sex Rituals, etc.
3) The fact that Constantine was responsible for "re-creating" Christianity