Arcane is useless now.
All the stuff you used to NEED (improved CS, Evocation, IAE) all mages get now.
The only thing worth getting now is POM.
Fire is more usefull in MC than one may think.
22 Fire/29Frost is Gnimish's current build. I get very effective damage vs. large groups, Flamestrike, Frost Nova, Blastwave, Arc Explosion 1-2 times, redo Frost Nova(Reduced Cool Down) and FireBall or FrostBolt the remaining. Its an excellent grind or PvP build.
Also is very effective in Instances. I ran UBRS yesterday, was 2nd in overall damage, I aggroed very little, and if I did I threw up Ice Block, others get aggro back and I can pound away again.
I only have about +140 damage or if I use my Fire Ruby, I gain an extra 100 damage on 1 Fire spell which I typically save & use with Blastwave.
I crit regularly with all my spells, Flamestrike & Blastwave give me multi crits usually in a single cast doing 1000 damage crits. My Fireball and Frostbolts crit around 1100-1200 regularly.
The only downside is if I don't crit regularly my damage is a bit low versus other more standard builds. Overall though I'm satisfied with it.