I've heard people mention the use of RAID before, and there's no doubt that such a system will work well, at least if you manage to ensure data is never accidentally erased. However, RAID also needs a lot more money and maintenance than other methods.

If safety is so important, I'd use the DVD method I mentioned above, and make two or three copies, not one. Store in different locations and forget about it. If you use decent DVD media, like Taido Yuden, you may not even need to check it except once a decade, by which time you probably will need to transfer the data to something other than DVD anyway.