I'm right now a 54 holy priest, heres my idea for ya. Go holy all the way till you hit about level 50. From here, take all your new talent points and switch over to Shadow and put 5 points into Spirit Tap. This gives you a 100% chance to gain a 100% bonus to your spirit after killing a target that yields experience or honor. Then switch over to Discipline and put 5 in wand and 3 in shield. From here you should be pretty good, you get a huge mana boost after killing something / someone, so it doesn't take as long to get your mana back. Then you get a stronger wand which is your last resort. As well a more powerful bubble shield. If you feel like stacking out the rest of holy go for it, otherwise go back to shadow and fill in S. Word Pain / Mind Blast / Fade for more DPS when you are solo'ing.