If the data is really that important, you probably want to build two storage servers, one to keep backups (unless the data is already stored somewhere else, and will continue to be available there). RAID won't protect you against filesystem corruption or accidental deletion, nor more severe problems like data centre floods (although unless you have two data centres, that's going to be difficult to mitigate).
You also have the option of archiving the data to tape, or even encrypting and archiving it to Amazon Glacier, just in case all else fails. I'm guessing you've considered these issues, but they're worth mentioning regardless.