It's all very well using an online backup service to provide peace of mind but the real test comes when disaster strikes and you need to restore from the backup. It's going to take an awfully long time to download 500GB of data over the Net. I see that Amazon do offer a service which allows you to send them a disk; they will restore your data to it and post it back. That's fine if you can afford the time, I suppose.
Bernard's proposal to buy a couple of disks and store them somewhere off-site but not too far away still seems to me to be a far better proposition, albeit that it requires a good deal more discipline in keeping the backups updated. Of course, if you live in Kansas City (I think it is), where you can get a 1Gb/sec connection to the Net, you might take a different view.