I am looking to add 1 more character to my setup to have a total of 5.

I currently have :

Warrior (Protection)
Priest (Holy)
Mage x2 (Arcane)

I want to focus on 5 man dungeons and then eventually raids.

I am currently able to do some dungeons however my main problem with boss encounters is running out of mana on my priest healing so much.

I think I can attribute this to not having enough DPS.

What class would you guys recommend adding to my setup? I sort of want to add a secondary healer that can both do damage and heal if needed.