Upgrade the pieces which scale the most first.
1. 3500 Weapon. You already have your weapon, which is the highest scaler. this is a good start.
2. 2250 Helm/Chest/Legs
3. 1750 Gloves/Shoulders/Belt/Boots (if you do not have Tyranical gloves, get those first for judgement 10 yard radius increase)
4. 1750 Trinkets. Ret relies heavily on burst, and trinkets provide lots of this.
5. 1250 Bracer/Cloak. while these scale the same amount as rings and neck, they also provide armour making them slightly better.
6. 1250 Rings/Neck. as above, these provide stats but no armour.
7. 1000 Meta. NOTE only buy this after you have reached Grievous Conquest (purchasing grievous armour for honour points) and put it in a spare helm.
NOTE: avoid pieces with hit rating on them. Gear has reached a point of providing you the hit/exp cap needed without needing to aim for it. a crit/mastery piece will be better than a haste/hit piece as a result. This mainly applies to off-set pieces.
It is only useful for defensive situations with the pvp resil it provides. for output damage, get the legendary meta or Strength primal diamond until you complete the phase of the quest.
In that is the case should I buy the Holy shield for extra healing effect, or go with the plain prot shield?
The intel shield will provide you with nearly no improvements to healing. The only thing Intel gives a ret pally now is spell crit, and not much at that. The Strength on the tank shield will give attack power, parry and spell power. Far superior.
In terms of the viability of sword 'n' board in pvp, most people are saying it is useless. This is still up for debate though. It will most definitely increase your survivability against a warrior or rogue, and should always be macro'd with Seal of Insight for the buff to healing.
Sadly, in all brackets you will need some level of carry. ret + mage + healer or ret + hunter + healer are the most effective. some are finding luck with ret + dk + healer or ret + warrior + healer but being melee heavy and lacking in CC means its burst or die.