I would buy high quality backup rated dvd-r or more preferably dvd-ram (if your hardware supports it) discs. Then archive the files using a combination of rar and par2 (mostly used on usenet). Par2 is a data redundancy scheme used on most alt.bin posts and works miracle even on heavily damaged archives. Burn at the lowest possible speed.

If you are concerned with having physical media with potentially sensitive material you could always encrypt it, but I would rather rely on the security of the physical location.