"Not backup, for sure. If you've got 500GB to store, a big part of it is probably things like .doc, .xls, and .pdf files. Try reading these from a few software revs back, and you'll realize that if you store that wodge, you'll also need to "archive" an old-enough PC..."
Actually what you would use is a document management system.
The way the compatibility works is for every type of document you want to read, the required software must be available on the server.
Need to read v5 pdfs, a v5 pdf viewer must installed on the doc management server.
Other than that it's a glorified database application that allows regular people (non-techies) powerful search options.
As for storing backups on DVD, even 500GB disks, no real point. Network appliances with up to large numbers of up to 1TB hard disks are more reliable, and a hell of a lot faster.
I use a 64 x 500Gbyte appliance as a backup/disk mirroring system.
The backup software creates a copy of each drive and stores them in various time slots. So I can retrieve a document from 5 minutes ago to 6 months ago, depending on when the oops happened.
Retrieval is as easy as sticking a '~' in the path name, eg. L:\Engineering\~whatever the path is, and copying it back to the correct location.
The appliance has fault tolerance built in, using wiz bang wizardry to automatically replace data into free space should a disk go bad.
Expensive but reliable stuff, and much faster than using optical media or tapes.