The Maintankadin website has a lot of good stickies that should explain a lot. A good gear link to start with is the Consolidated Gearing Guide sticky. I also suggest reading up on the WotLK Mechanics sticky to explain a lot of the theorycrafting behind why you should be using certain threat rotations and gear.

And, yes, 535 defense is the min for heroics (5-man) because the bosses are only level 82. You'll need 540 for any 10-man/25-man raiding.

EDIT: Several edits. Forgive me, not feeling well today.