flamestrike, sad to say, is junk. don't use it.
arcane explosion is ALWAYS better than flamestrike, and even for a fire mage blizzard is sometimes superior.
for my mage, when stuck in fire spec, i'd open with pyroblast, then use scorch or fireball, depending on how much health the mob had left after the initial whammo.
in my opinion? stay frost. you can PvP as 17/0/44, dps in 5-mans as 10/0/51, or drop raid-worthy bombs as 40/0/21. fire is nice, but it's not *better* for anything except (arguably) ultra-high-end raiding.
Yea, I did actually go back to Frost after a couple levels. It's just too damn fun, and there's so much synergy amongst the spells and talents. I'm up to 68 now.
I've been thinking about respeccing to Arcane just to try it, but I'm specialized in Shadoweave Tailoring. Not that that's much of a hindrance, of course.