Redundancy is key. I like DVDs because 1) they're cheaper per meg (significantly so) and 2) I'd have a huge pile of CDs if I wanted to store all my photos on them! Even more so since I shoot raw now.
I burn them to two discs, one that I keep loose with other backup discs, and one that goes in the fire safe. I always tell Nero to verify after burning, to make sure that the discs are 100% error-free, at least to start with.
I also am trying to keep them on one of my other PCs that I use as a NAS box, but disk space is an issue there, too!
Keeping them on multiple hard drives is better than just one, but I'd want at least one copy on DVD no matter what. If you're really paranoid, create something like PAR files or SFV files for the DVD's photos, so you can easily check to make sure that each file is error-free in the future. Check every year or so, then rest easily.