Are you talking about standard DVD cases or jewel cases? If it's the former and you want to hang onto the packaging, you might want consider something like this:
http://www.bagsunlimited.com/cart/browse.asp?subcat=220
The Reader's Digest size fits TV Guides (the old size, before last year's revamping) and are also a perfect fit for standard DVD cases. They're economical, especially when bought in bulk, and pretty tough. I haven't actually tried stacking these with DVDs in them, but I also use them for VHS tapes and can stack 2-3 deep with no problems.
I haven't quite got enough stuff yet to start storing DVDs in them -- but I'm getting there. I'll keep the new or frequently-watched items out on shelves (I use a spinner tower for a lot of storage and display with minimal footprint), and eventually older, less-watched titles will go in the boxes.
If you're looking for storage for jewel cases, something like this might work for you:
http://www.bagsunlimited.com/cart/browse.asp?subcat=74
Hope that helps!
Mary