Enhancers are things that affects your spellpower. This includes: staff of foo (like staff of fire), ring of foo, robe of archimagi, wild magic mutation, !brilliance, some spells.
Spellpower affects most things of spell. For conjurations for example, it determines how much damage the spell does and his accuracy, plus duration and size the spell has those variables too.
For example, bolt of fire:
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does six dice with a maximum total damage of (18 + (power * 2) / 3). At power 50 it does 6d8 damage; at power 100 it does 6d14
and
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To-hit: 10+power/25.
So generally speaking, more spell power means stronger spells, which is very good.
By the way, spell power is given by:
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(Spellcasting/2 + 2*avg school skill) * INT/10 * (1.5 ^ enhancer count).
but that shouldn't mean you should overtrain magic school after % spell failure get to a decent value, because on spell power there are a lot of cap, so it gives very bad returns. At most, you should use enhancers when you have them.
That said, you shouldn't follow very close TomberryJam's advices. Most of them are bad or outright false. Getting 2/3 enhancers by Lair is totally fantasy, and actually most of chars won't find (or use) more than one or two in their entire game.
You shouldn't Air for conjure ball lightning and Tornado. Bolt of fire and orb of destruction are enough to clear the game. Sure, with De's apts you may go for them if you want, but this is totally optional.
Being spellpower good and enhancers good, most probably you should use staff of fire over staff of power when using fire spells - as you should get more benefits from casting fewer but stronger spells than more but weaker ones.