How-to: Design, build, and configure a (low-cost!) Backup/Server Monster

My company needed about 1 terabyte of backup storage space. We use software that keeps every file that has been changed or created (incremental backups + sessions for all you geeks). This requires about 400GBs of storage for every 100GBs of data the company uses if we want to be able to keep one years worth of files (milage will vary depending on how many files are changed, and the size of the files). We built a system that can store two years of changes to every file ever created easily! Unfortunately, the problem with this is (of course) that if we get incriminating emails, and then they "mysteriously disappear," it doesn't look too good for us.