Put simply,
AGI is responsible for your power and avoidance.
INT is responsible for the damage of your spells.
STR determines your melee damage.
I have found that AGI is not important and INT and STR are the stats you want to improve and in that order.
Don't completely ignore your WIS, and this goes for any class, because you need it for resistances and a few more points in this stat can make a big difference.