I have to agree with the others, there are two different setups to use. Both requires atleast 2 tankers, 2 healers.
2 Tankers - Prefered if they are not the same class.
3 Healers - Also here try and get some different, we found it very usefull to have 2 paladins in the raid (not needed to be healer, but 2 paladins are great for buffing.), for 10 % BoK and one other doing BoW and BoM.
5 DPS - I suggest you bring 2 melee dps and 3 casters for this setup, since on some bosses to much melee are a problem, and vice versa with ranged. Allways good to keep dps classes that can buff the raid, for example a Shaman of any sort so you dont go with 3 warlocks and 2 rogues etc. Try and balance it.
Pros with the setup : You have alot on healing power here and this is the suggested starting setup for Naxx raids, The tankers will be taking alot of damage since most just burst in here straight after dinging 80. There are if i remember corrent only one Boss in Naxx with an enrage timer, and thats Patchwerk and with 5 dps it shouldnt be any problems to get him down before that.
Cons : If your DPSers are not good enough and slacking around on 800 DPS you will experience problems with Packs (The gargoyles for example.) and Bosses, you do have 3 healers for some extra healing, but aswell if the boss do not go down they will eventually run oom. Three healers do not mean immortality.
2 Tankers - Same as above, try not have the same class.
2 Healers - Now its important to now have 2 of the same class, due to every healer has its weakness. (will explain it later)
6 DPSers - Same as in Setup 1. Try and not run with 6 melee nor 6 ranged. Get a nice balance.
Pros : You now have 1 more DPS than in the previous setup, and by simple math the bosses and Packs should go down faster than with only 5 dps. The thing with this is that Healers do not require to heal for the same amount of time as they had to with 5 DPSers, on the other hand they are one less to heal the same damage during the time of the boss encounter. Since the boss will still make the same damage as if you had 3 healers (quite obvious eh? )
Cons : You now only have 2 healers, wich can make the whole thing Complicated, now its more on the tankers gear than anything else, the tankers will be taking tons of damage in the current gear they have from leveling and a few heroics.  So if the tankers are to bad geared there is no way you can do it, even though your healers are really good, they wont be able to make the spells cheaper, and trust me, bosses do alot of damage when the tankers still using leveling gear and T6 etc. Aswell as its depending alot of the healers stills, they need to maintain a high HPS aswell as keeping their Mana up.
Personal experiences : We have now cleared most of naxx, and in our first raid we started off with 3 healers 5 dps and 2 tankers. It worked fine during packs, we even survived ninjapulling 2 packs at once and still survived. So the Packs were no problems. Though at the first boss our paladin healer DCed (Noth the Plaguebringer). We waited for a while then decided to go try the boss 9 man to see if there were any changes from 40 man naxx. And there were not alot of changes on Noth from 40 man naxx. Anyhow, we went in there and did the boss without a single death, 9 man. With blue geared tankers One prot paladin (he had some t6 though) And our offtank a Deathknight. We used 2 druid healers for that fight.
This seemed to work, and we continued clearing packs now with 6 dps and 2 healers. We took down some more bosses without any major issues, had a few whipes before we got taccs, and we had to take a manapot now and then. This worked untill we got to Patchwerk, then our fortune stopped. We had no problems keeping our paladin tanker up on Patchwerk nor anyone else, exept when he turned for our DeathKnight. Patchwerks damage were simply to much for us to outheal, he did 10k hits on our Deathknight multiply of times, our Deathknight had 26 k hp and he still went down as butter due to lack of parry and other important DK Tank stats.
So by this little story, i just want to say that 2 Healers DO WORK if the tankers have the gear for it. If they dont, there are limits. So for first timers, take 3 healers unless your some previously sunwell raiding guild.
i don't aggree in only 2 healers. At least Sapphiron requires 3 in this state of gear and it makes it quite difficult even with 3. I was not able to solo heal our tank in swp gear and some heroic naxx gear as a holy paladin in 10 men. It was a pain and i was oom ( no retri paladin or spriest around ).
But that is another thing ... holy paladins suck big time at the moment ...
Even though there is no PARTICULAR raid composition that is needed these days for raiding, now that the classes have become more alike, I still like this setup the most. If need be, and you need a third healer, you can drop the Ret Paladin for another healer, but once in the gear, it is definetly the group I want to use. ;D
Would be my personal preferance of a solid setup, you have a lot of buffs enhancing people's individual strength, and you have some people who can change possible roles during the fight (myself being a priest I tend to choose for feral droods because innervate owns bigtime)