The soft cap for def is 535. this allows one to run heroic 5mans with out being crit'ed. the poster above was off a little. the 540 def is to keep from being criticly hit by a lvl83 bad guy. i had on mostly blues when i started heroics. i believe most were rep pieces as well as some BS crafted pieces. i may have been 3 or 4 points short of 540 when i started heroics.
the thing i did was search the interwebs for preheroic pally tank gear. (dont remember the site? wowwiki maybe?) used that guide to build a set. if you google wowtankadin, you should get some hits on sites.
tip. Def cap first / Stam gems are your friend / Dodge is your second best friend.