I would've suggested tape drives, but they're really expensive. May I suggest DVDs or even Blu-Ray discs? The problem is that you won't be able to fit all your data into one disc, and thus you might have problems locating backed-up data in case you need to recover.
In my opinion, the best solution to cheaply and regularly backup your data is to use another hard drive. DVDs or Blu-Ray are good for a one-time backup, or you can use RWs for repeats but this takes time. Also, I don't like optical media for reliable backups. Tape solutions cost too much money.
You can buy another hard drive to cover 400gb for not much more than the DVDs it would take to back everything up just once.