These artifacts add to your regular spell power rating. Higher spell power causes more damaging or prolonged spell effects.
For damaging spells, such as Fireball, the spell being cast has a certain multiplier which is multiplied by the caster's spell power. The result is the damage done to the target.
For continuing spells, such as Blind, the spell power value indicates how many combat rounds the effect will last.
Arcane Necklace of Magic: spell power +4
Witch's Broach of Magic: spell power +3
Mage's Ring of Power: spell power +2
Caster's Bracelet of Magic: spell power +2