Protection works like armor, so it has dimishing returns in that the more you have the less you gain from increased protection. So while I'm sure there's a cap somewhere you'll hit the point where it isn't worth it to increase it anymore long before you hit any sort of hard cap.
I still have my culture armor with +prot gems, that and the right rings, cloak and necklace with my AAs which works out to a little better than 40% magical damage mitigation. I think my total protection across the board is somewhere around 1200 without running Mental Barrier but I don't remember, it may be higher or lower, sorry
Unholy damage is used more often than holy damage in both PvP and PvE.
Louhi does holy damage...gearing for only one fight is unadvisable, however. So general protection is much more favorable! Rings cloak necklace and T3 gear FTW! If you are so inclined, the Ibis weapon can also help with protection.