This is an image we have in the wow-europe mage forums:
http://img241.imageshack.us/img241/4113/magetriplepointlr0.jpg
Pretty much, if you have that spell damage & that crit, you choose that spec and your DPS will be maximized. There are some solid mathematics behind that, but I for the life of me really can't be arsed to read them.
And Brandon, fire without Flame Throwing is suicide, specially on Gruul. Trust me.
As for full Arcane, it sucks. I am full fire, with about 1100 (fully raid buffed) spell damage, and this other guy is full arcane, beats me how much spell damage, but it's aroundish mine, and when I died when Gruul had half the HP, while he survived to the end, I still had done more damage than him. That's lolable.