I realize that most have stated a 600 or higher number for HP, and I'm not going to say thats a bad baseline. However, I do want to point out that I have successfully Tank Horoth with 550 HP 3+ times, with one of those being on Hard, on my first life of my Paladin(DoS tier3) with a single healer for normal and dual for the Hard run. It does take some equipment though to help with this. Things like World Shaper's cloak (I would still recommend this for 600+ as it gives you a safety net), healing amplification (enhancements as well as gear). Also I was sporting an 82+ AC.
Most of the time I was religated to being the Sulu tank even though I was a human. I have come to believe that the amount of hit points needed is not as high as some think, but each point above a worst case 600 disintegrate is always good.
I would also point out that I did this Sword and Board which is a much different technique then TWF or THF and requires the art of know how to use Shield Blocking to avoid some of the hard hits. And yes I was able to keep agro of which ever I was tanking through use of Intimidate and hate generation and other enhancments available to Paladins for generating hate.
As a healer with over 30 runs ranging from Normal to Elite - I still prefer the high AC/Heal Amp model tanks over the low AC/Heal Amp w/ Extra high HP models. Reason: Stuns can happen, the HP buffer is only as good as the last heal proc. Where the AC reduces the chance of being hit during that time frame.