There's a really awesome website that I use whenever I'm deciding on gems called WoW Gem Finder. I was looking back through my Holy Priest Healing Guide, and realized that I never mentioned enchants or gems. Enchants for another day, but I'll hit up gems now. I highly recommend checking out elitistjerks for gem suggestions if you are interested in the theorycrafting behind choices, though they have not updated with the new epic gems on the linked post - it's likely you will need to read through the thread.
However, I'm anal retentive, and cannot bear to lose a socket bonus, which EJ often ignores. Therefore, I always use a matching color for the socket. I find it handy to know what stats are related to which colors, so here you go.
You can see, just by looking at the color -> stat conversions that Red Sockets are for spellpower, Blue Sockets are for regen via spirit/mp5, and Yellow is for all the fun stuff. I've italicized the stats that are generally useless for priest gemming in PvE raid healing. You could argue that stamina is a good choice, but unlike the days of Primal Mooncloth sets, we don't really need to gem for stamina anymore, as we get plenty naturally from our gear.
For blue sockets, I always choose Spirit over Mp5 due to Spiritual Guidance. Maybe mp5 is actually better, and if I wanted to, I could do some maths, but like, math is hard, so... no. Spirit it is (see, this is why I recommended elitistjerks!). I never use a solid blue gem for my blue sockets - if I need more mana, it's better to use Intellect, at least for raiders. For mages and warlocks who are determined to take advantage of socket bonuses as well, spirit may be a better choice for you too, due to talents boosting spellpower through spirit.
For Red, of course I always choose spell power, since there aren't any other good choices.
Where you really start to see differences among priest choices and gear levels comes in the yellow socket. When you are first starting out, always pick Intellect for more mana and mana regen. Once you have a solid mana pool and no longer need to panic about your mana sustainability (which, for the record, is a lot easier in 25 man raids due to more buffs), you can start picking up crit and/or haste. I try to balance between the two stats, but plenty of people have a preference for one over the other.
Now that I have so much intellect, mana is rarely an issue for me, nor is crit. I prefer to gem for Intellect over crit, since you do get some crit boost from intellect. However, I do not necessarily recommend this if you really do need a lot more crit.
I've listed my top choices for each color socket. The first gem is the epic, the second is the rare gem equivalent. If you are not yet ready to blow large amounts of gold on the epic gems (new to raiding, doing heroics, 5 mans, etc), pick up the lesser gem with similar stats.
In reality, it is unlikely I would use any gem that has spirit on it in anything other than a blue socket, but you get the point. Creative combinations and careful shopping can save you lots of gold on gems, and potentially net you better rewards. For Blue Sockets, I always use a combination gem. For Yellow sockets, I always use a solid yellow gem. For red sockets, it depends on the current price of Runed Cardinal Rubies.
In short? Don't gem for spirit or mp5. If you need to use a blue gem (socket bonus, meta bonus, etc), use a purple or green gem that includes spirit. If you need mana regen as a raider, gem for Intellect (this is slightly less true for those who focus on 5 mans - the more people in your group, the higher the chance you will receive more benefits from intellect regen opportunities). Once you have enough mana, pick up crit or haste. Always gem for spellpower. Combine multi-colored gems to save some gold.
For your Meta Socket, the following are the only real options for a Holy PvE Priest:
Beaming Earthsiege Diamond - Easy socket requirement to achieve. If you are in need of a lot of extra crit, this is a pretty good choice.
Bracing Earthsiege Diamond - Threat should NOT be an issue for you. If it is, there are more important problems to address. The only time that threat may potentially be a significant problem is in 5 mans with a fresh tank who is learning the ropes. Therefore, this is ONLY a decent choice if you are with a group just starting 5 man heroics, etc.
Insightful Earthsiege Diamond - My favorite meta. The mana restoration proc happens quite a bit for Holy Priests, as we are often using lots of instant heals mixed in with quickie flash heals.
Ember Skyflare Diamond - Always a solid choice. Insightful meta is better until you build up solid mana reserves, at which point this becomes a great meta, particularly since it scales so well with gear.
Revitalizing Skyflare Diamond - Don't be bad. Blizzard put this here just to trick you. Just say no to "increased crit heal effect" which half of the time translates into "increased overheal amount and therefore a complete waste NYAH NYAH". You canNOT rely on crits when a heal is needed, and "increased critical healing effect" is very different than increased spellpower, which is RELIABLE. Also, I recommend Spirit or Intellect over mp5.