[Taqwus]

Interesting, a Ring of Wizardry will boost Unholy priest power (at least on a Mound Fiend it will... just checked), although it will not boost Holy power. This is obviously rather expensive, however. A Ring of Sorcery will not boost Unholy.

Apostasy, if memory serves, is once of those 'magic resistance negates easily' spells. It'll help to have Communion boosting and also to have penetration items -- Spell Focus, Rune Smasher.