Personally, I wouldn't use optical media. Even if you disregard the media itself, you still need to maintain drives that can read them. It's also more expensive per GB than hard drives. That's my opinion tho. I'm certain there are advocates out there.
Currently, hard drives seem the most cost effective media for long term storage. There are other mediums that are viable, but upfront costs are higher. With external hard drives so cheap, I would look into maybe buying a couple of 500GB external drives and simply keeping a complete copy on each drive. Every now and then it would be prudent to connect them and verify everything is still in order. There really are no hard and fast rules to this. Generally, the more important the data the greater the effort to keep it safe. Many people go so far as to make a second backup and maintain it at a separate location or in a safety deposit box. This can help prevent data lost in the event theft, flood, or fire.
The good news is that it is relatively cheap to duplicate the amount of data you are describing. For myself and others with file storage needs on the order of many terabytes, it can get expensive to maintain proper backups.