It appears that the door will house dvd's as well. If it is sagging now under it's own weight, it will be even worse when you load it up.
I'm guessing you're getting some sag/twist from a couple different directions. The weight of the door is pulling the top of the base unit forward and pulling it out of square, which allows the non hinge side of the door to droop, in addition to the door itself sagging a little.
As was previously suggested, a few shelves would help the main unit's rigidity and take out some of that twist. It would also help if you could gusset the corners of the door. A triangular piece in each corner that attaches to the side and bottom/top. A piece that goes all the way across would work as well, but you want to avoid adding too much weight. A thin piece of 1/4" would work. You are just trying to provide some sheer strength to the box.
If you go the route of adding some pieces to the edge of the door, you can "pre-rack" the door up a little so it sags back to square.
Last edited by Schu; 03-16-2011 at 11:40 PM.