Having a priest around will help quite a bit with AoE grinding, particularly as you're still learning it. Most people recommend L.40 for AoE stuff, simply for the mount. Makes it alot easier to round baddies up. With a priest's shield and heals, however, you might be able to get away with it even earlier.
Regardless of when you start, speccing into frost is a must for the most effective AoE grinding. Again, with a healer along for the ride, you might be able to hang in as fire, but it will still be less effective than frost.
As for single-target grinding, I peronally don't even have Arcane Blast on my hotbar anymore. It seemed like a cool spell at first, but its mana inefficiency really kills it. If you like the quick-cast nature of Arc Blast, try Scorch instead. Ridiculously efficient and very respectable dps if you spec fire. It should never be your pulling spell, but it's a good one to use in the middle of a fight with a melee guy on ya.
All that said, healer/mage is in my humble opinion one of the poorer teaming combos. The couple of times I've done kill quests with priests, I kinda felt like I was doing all the work. With single pulls (especially at 60+), most mobs are dead before they get very many hits on me. So my biggest problem is mana, not health. I always found having a tank (especially a pally with their AoE tanking) or another dps'r to be greatly preferrable. With that in mind, consider having your friend go shadow (or disc? dunno much about priest talents)--that way you get dps and a mana battery...though I'm not sure when spriests get the ability that helps party members regen mana...
On the other hand, I know a couple holy priests with whom I get along very well...so in those cases I rather like feeling like I'm doing all the work b/c I know what a PITA it is for them to grind and I enjoy feeling like I'm helping them. But that's not really relevant here.
edit: spelling
Edited, Oct 26th 2007 9:55am by AynLoD