If you want online storage, I'd recommend Jungle Disk. They're a front-end to Amazon's S3 service, and they're dirt cheap - 15¢ per gigabyte for storage, and 20¢ per gig transferred. . 50GB would cost you $10 to transfer it up, and then $7.50/month. Best of all, you wind up using Amazon's backups, which are quite a bit better than anything you devise yourself.

They have Mac, Windows and Linux clients.