Tape is quite commonly accepted as the most reliable way of backing up data, while CDs are unreliable at best. CDRs are susceptible to degrading if exposed to (UV)light, but even when kept in a dark place, they can still fail easily within a year. I have CDRs which I burned well over five years ago still working perfectly, but others have failed within months. A backuptape, if stored under good conditions, can keep its data safe for many, many years.
Your best bet for keeping long term backups is making several backups on different types of media. One backup on an external harddrive is useless. One backup on CD/DVD ditto.
I make a backup of my most important data (about 7-8 GB) once a month. I copy it to a different computer, which copies it to an external harddrive and to DVD. This gives me the original files and three backups on different independent media: one on DVD, one on the external harddrive and one on the backupcomputer. If any of the backups fails, I still have the others, including older backups made in the months before.
Make backups. Your computer _will_ fail eventually.