High priest gear setup

[Ptah]

Well, in addition to what Aedra said i'd only add in few coins:
Head:
Pope Mitre (-10% cast time) - Pretty nice bonus, along with nice def and mdef bonus for being HP
Also, things that wiz usually get for int work (Isilla carded Valk Feather Band, Isilla carded Lif Doll etc etc)

Armor:
My strict advice would be Orleans, unless you do accessory switching to acquire interruptible cast.

I think, divine cross+Spiritual ring+clip/rosary is awesome for boosting damage. All you get from that combo:
+7 dex (+4 dex from Divine cross, +1 from ring, +2 from combo)
+2 int
25% demon/undead resist
+25% MATK
+50% damage with heal against undead
+30% ME damage (Feel it? 25% matk boost AND +30% ME damage. Just ouch!)
SP recovery +9%
(Add in rosary/clip and it's card bonus)

Another nice option is Priest card set, which is:
Geographer, Rideword, Enchanted Peach Tree, Cookie & Seal Card

It offers:
LUK + 2
+10% damage with Holy Light
2% chance of casting Heal on yourself with each attack (remember, those DO NOT work if you're in aftercast delay)
2% chance to bless yourslef with each attack
+2 int
+1 Mdef
+10 Hit
+3 Flee
+9 Crit on demon/Undead
VIT + 10
-10% cast time
30% demon/undead resist
5% more experience from demon/undead

But note that is occupies Accessory, Shield, Armor, Headgear and Weapon slots, and you'll be strict to only those gears you've put cards in.

In shortance - you get to choose between survavibility(card set) and more attack power(Divine cross).

Card in clip/rosary is totally your choice, i use a variety of cards depending on situation (even a popo card in nameless to get an ultimate healer~).

Also, your build is once again hardly binded on what you'll want - to survive or to hit hard.
Just remember - int not only increases matk, but provides heal bonuses, along with better SP regen. For some reason i always ended up with ~90 base int (120 total), ~80 base dex (110-120 total) and rest ~50-60s in vit.
+Don't forget you have perma bless on yourself, and count it in your calculations.