Everything Jrizz said is accurate.
To clarify on a few points:
- While it's possible to have more than one prestige class, it is not mechanically possible (in NWN) to have more than three classes, prestige or otherwise.
- While a character can be a follower of multiple deities, they will not gain the advantages of being a worshiper of any one of the deities (ie. clerics of their deities will see them as being Neutral towards them.)