I personally think that Arcane/Frost mages own, and you don't even have to put many talents in Frost to make them do that. With a Arcane/Frost build, you'll end up have a base Crit Chance at like 15-20 percent, and then I forget what Arcane talent but one of them adds 30 percent to your Crit Chance, and with Frost you can get 50 percent Crit chance if the enemy is frozen. 20+30+50= 100 percent! And you get all those talents that add like atleast 150 percent more damage on Crits so, I'd say don't go for just 1 of the 3, go for like 1 big, 1 small.

I heard that the crit ratings dont add up so....

the crit with that is close to 70%
b/c 50
% on frozen then you multiply some hot the 30% and your own crit to get it