Actually, for a tank, you might not want the highest AC possible. That may sound counterintuitive, but high AC just gives the opponent that much more of an incentive to ignore you and go for juicier targets. Going for lots and lots of HP and some DR while still remaining reasonably hittable, on the other hand, will give your enemies incentive to focus on you more. In the absence of a true aggro-drawing mechanic (the Goad feat doesn't count...), a tank doesn't want to be a turtle.
Oh, and I totall recommend light armor over heavy armor any day of the week for a tank. You want to be able to get between the opposition and your party's squishies, and to do that, you need to be mobile. I'd also recommend Anklets of Translocation or similar, to be able to help out where you're needed.
A word on DR: many people are of the opinion that DR isn't worth it for a PC. To a degree, that's true, since sufficiently high DR to have an impact at mid to high levels is extremely costly. Crusaders get more benefit out of DR than most classes, though, since it pseudo-stacks nicely with their delayed damage pool and their self-healing capabilities (tack on a vampiric weapon to suck even more HP back...). Appreciable DR can be found in the Mineral Warrior template, for example: 8/adamantine is nothing to scoff at.
On the other hand, Crusaders have some pretty good maneuvers which grant short-term DR, so you might want to just make do with that.
