You cite valid issues with backing up, trusting yourself, etc.
One of the most important questions to answer is how to restore.
If you are backing up to some on-line service, how do you get your data back?
What happens if this start-up service goes under?
Personally, I burn CD's of just the data. I know I can re-install the O/S and applications. So quicken data, e-mail, correspondence and photos are burnt to CD's periodically AND stored in my safe depost box for off-site storage.
I also store data on-site on a mirrored volume on another server, and that can be done "hands off"
If I had just one PC and was really worried about my data, I would get one of those removable drive bays and buy two extra drives about the same size as your current drive or larger.
I'd keep one in, and one out, and set up automatic copies from your boot drive to your back-up drive and swap them weekly, monthly, or whatever.
I just wouldn't trust another party with my data, certainly not a startup.