What two Talent Specs. should I have on a WoW Priest?

I have just reached level 30 on my WoW Priest and I am wondering what two talent specs. that I should do. Up to now, I have been a Disc. Priest and I mainly heal (I have only DPS in dungeons a few times). I only ever do dungeons (when I heal) or PvE, I rarley do PvP as I am not very good at it. I am wondering though, what two Talent Builds should I do if I mainly heal in dungeons and PvE, Disc. and Holy is the best healing, but should I go with Shadow and something else incase I need to do DPS? Thanks a lot for answering!