Both Frost Warding and Arctic Winds sucks. Don't take them. And Imp. Frostbolt is an absolute must. As well as Emp. Frostbolt, if you're going frost.
The build ranzo suggested is the best imo.
Anyway, for leveling, fire has the advantage of killing them before they reach you, but accidents may happen. Frost is good because it slows them down, so you can also kill them before they reach you.
Well, the best I'd suggest is that you go fire up untill level 45-50 (because that's when frost begins to be interesting), then go up to 70 as frost. Then you decide, since you had experience from both sides.
Arcane is interesting, but it's more of a support tree. It really doesn't give that much DPS.
--Edit--
Btw, you'll probably have the same doubt I had. When the +dmg items start appearing, don't hesitate to take them. +dmg is truly what gives mages power, screw it if you get less mana.