A good question.
The combo artifact gives +20% to the power of all spells and +10% fire, magic, poison and astral resistance.
In general, 20% power is like +4 intellect (4x5%), but as a mage, it is better to stack up on intellect rather than spell power due to the 7-point bonus of 15% (as you know each 7 points of intellect grant an additional +15% spell damage bonus). The resist bonuses are nice, just be sure to dismantle the artifact if you need magic or astral resistance in the upcoming battle.
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