Reputation is maxed out at 999/1000 exalted. You can't get higher than that, at least not in the current patch.

Currently you will receive 75 Honor points for each additional stack (20) of Runecloth. So depending on where you stand in the realm of Honor with Darnassus, to go from Revered to Exalted requires 21,000 points which equals 280 stacks (20) of Runecloth. I recommend doing as many quest while your leveling along the way that will give you honor points toward Darnassus, because buying the runecloth can get expensive if your waiting till the last minute to boost your honor. Of course you can always stop what your doing and go back and do all of the quests! :)