If your really worried about your system and your data there are a couple of things you can do.
Regardless of what form of drive you have - ATA / SCSI / SATA / SAS / SSD.
With two drives you can "Mirror" the drives, so if one drive fails the other keeps going. (Never "Stripe" two drives unless you only want speed)
With three drives you can "Stripe" the drives - two drives contain your data - one contains a "Parity" bit - which means if one of the three drives fail, it can be rebuilt from the other two.
Mirroring requires 50% more storage then you actually get.
Stripping + Parity requires at least 1 HD more then you actually get.
More complex methods do exist - I have a dedicated NAS that uses 4 drives, and at work I maintain a couple of SANS that have 12 or 16 drives.
However - this only provides hardware redundancy - not backup capability.
For full coverage you should have the following.
A Live System / Data Set - your Computer.
An Incremental backup of your Live System / Data Set - often to a NAS.
An Offline Copy of your System - updated at fixed points in time, or before major updates. (Sometimes this can be a rotated incremental backup)
Unless its in three "Digital Locations" its not truly safe.
The offline copy should be located somewhere other then next to your computer.
You can further increase this with Cloud Storage (Assuming you trust it..)
Dropbox lets you sync documents, as do many other services - Wuala, Google Drive, Sugarsync etc.
There are also solutions that let you backup online - I use Crashplan, which backs up my machine incrementally to both a local copy within my house, and to the cloud automatically. (I paid for the latter option)