I've been saving my original purchases, mostly in the event I ever really purchase a Kindle (although it's very unlikely). Books are then added into my Calibre Library, and that library is backed up on 5 hard drives between 3 PCs and a netbook. It's also in 3 places in cloud storage. I really should just delete all original purchases at this point, but ebooks really take up so little room, another 4GB gained if I do delete the original purchases isn't going to matter in my instance.
As far as DVD storage goes, the quality of the media used is the important factor in how long they'll last. And of course don't expose them to high heat, humidity, handle only around the edges so you don't get oils and acids from your fingers on them. Burn at slower speeds for best success. I've got some close to 20 years that still play just fine.
Here's a good read about blank media quality to use for reference if you're unsure which to buy. I only use Verbatim DVDs and I have never had one fail yet. I bought some Memorex on sale once and never again, a lot of them wouldn't even burn correctly. I just wait until I see the Verbatims on sale and buy a 100 spindle.