Dexterity And Strength

Anyone know how these apply with spell damage? Specifically, does it stack with spell%? Like, say you have +100% and have either +100% strength damage or +100% chance to crit, would that be doubled again, or add another flat 100% to the damage?

Or do they both behave differently? anyway, curious to know, mainly as a mage, what i should invest my points in to get more damage, or if its a waste of time and should invest in defense and agility.