540 pre Heroic gear isn't really that hard at all. At least not if you're willing to spend some cash which you should have if you quested.

Here's my warrior's armory link

Zullga

The only piece of Heroic gear she is wearing atm is the gossamer's trinket which doesn't add any defense, and as you can see she is actually over the Heroic def cap by quite a bit. I actually working on switching out some defense for more stamina.

I'm not sure about shamans but as far as tanks go this is generally what I've been told for Heroics.

22-23k HP
20k +AC
535 Defense for Heroics 540 for Raids
Dodge and Parry = 30% (in some combonation, although this may be more for warriors than for paladin or dk)