You have many choices. One, you can pay for extra space at $1/GB/month. This includes backups of all your site data, and use of our content delivery network, as well.
Two, you can use Amazon S3/EC2, and set up a WordPress plugin that allows you to use them to host some of your content. ServerGenie allows this, but can't support the Amazon side of things.
Three, ServerGenie has NAS, FTP, and Cloud storage options that can handle this for very reasonable rates.