I thought I'd post this for giggles as my "moving toward end game" wish list for cloth items.

This set is particularly suited to a "holy/disc" priest (as opposed to a "shadow/disc" spec). While not the ultimate, it's what I am looking to acquire from this point forward.

Devout Armor set

The first major set a Priest can get is the Vestments of the Devout set. The set items require level 52 - 58 depending on the piece. It will take at least that long and likely to 60 (unless you have a large guild helping you out) to collect the full set.

Locations to find set items

Devout Belt - MOBS in Blackrock Spire
Devout Sandals - Maleki the Pallid in Stratholme
Devout Bracers - Crimson Priests, Thuzadin Necromancers and Thuzadin Shadowcasters in Stratholme
Devout Skirt - Baron Rivendare in Stratholme
Devout Gloves - Archivist Galford in Stratholme
Devout Mantle (Shoulder) - Solakar Flamewreath in Blackrock Spire
Devout Robe - General Drakkisath in Blackrock Spire
Devout Crown - Darkmaster Gandling in Scholomance

Set Bonuses:

3 pieces: Increases damage done by Shadow spells and effects by up to 7.
5 pieces: Increases damage done to Undead by magical spells and effects by up to 14.
6 pieces: +15 Shadow Resistance.
7 pieces: +20 Spirit.
8 pieces: Increases healing done by spells and effects by up to 55.

Total Armor: 491
Total Stats bonuses:
Agility: +12
Intellect: +134
Spirit: +115
Stamina: +65

Next up is

Prophecy Armor set

The next major set a Priest can get is the Vestments of Prophecy set. The set items all require level 60 and are found in the Molten Core Raid instance.

Locations to find set items

Girdle of Prophecy - MOBs in Molten Core
Boots of Prophecy - Lucifron and Shazzrah in Molten Core
Vambraces of Prophecy - MOBs in Molten Core
Robe of Prophecy - Golemagg the Incinerator in Molten Core
Gloves of Prophecy - Gehennas in Molten Core
Pants of Prophecy - Magmadar in Molten Core
Mantle of Prophecy (Shoulders) - Sulfuron Harbinger in Molten Core
Cirlet of Prophecy (Head) - Garr in Molten Core

Set Bonuses:

3 pieces: -0.1 sec to the casting time of your Flash Heal spell.
5 pieces: Improves your chance to get a critical strike with Holy spells by 2%.
8 pieces: Increases your chance of a critical hit with Prayer of Healing by 25%.

Total Armor: 584
Total Stats bonuses:
Intellect: +168
Spirit: +114
Stamina: +113
Fire Resistance: +34
Shadow Resistance: +24