Don't play the game as a mage - you will get bored quickly!

[Handel]

Once you start to feel the offensive spells are doing little damage you must upgrade them. The difference is huge and is much more then you can get by raising the INT.
At first glance raising the INT seems better choice because it gaves 10% increase. But this increase is calculated on the basic damage and the basic damage in the center of the 1st lvl fireball is only 120 points (I think) at 0 INT. So +1 INTgive just a measle +12 points damage.
At the same time upgrading the fireball doubles and tripples the basic damage which means it doubles and triples all the bonuses from the INT and from the destroyer ability (and from items if any).