After vehelpfulguy's doom-laden explanation of the fragility of hard disk magnetic media for backup purposes, I do hope nobody scares you further with the projected shelf-life (real, not manufacturers figures) of some optical media (CDs, DVDs), especially those at the low end of the market.
As to the addition of another hard drive, it will depend on your mobo (does it support IDE, SATA, or both and does it have any unused drive connectors) or you could even install a controller card.
Which mobo do you have?