There are archival gold CDs and DVDs with Phthalocyanine dye that will probably outlive you if stored properly. Decent quality cyanine based discs like Taiyo Yuden should last over thirty years. Even El Cheapo discs last a surprisingly long time if kept in the dark.
My 98 Nissan pickup came with a CD changer and I made six CDs for my wife to put in the changer. I listen to the radio and no longer have the wife, so those CDs are still in the changer. After 9 years in the Florida heat and humidity with no garage they still all play perfectly. The ATIP shows they were made by CMC, which is the ultimate cyanine based cheapie. Keeping them from light is the biggest factor.
A single HD, DVD or CD is fine for backup. For archive where you are removing them from the main computer you need redundancy.
For CD or DVD I think two copies on different media stored in different places in the dark is fine. I find the combination of external HD and CD/DVD handier since it is easier to back up until you can archive. I recently got a 500GB drive with an eSATA external case for under $150 delivered. That is 30c/GB including the case, so it would be less than 25c/Gb for the next drive to go in the case. And of course cost per Gb will go down with time. External HDs have become very cost effective but you don't want all your eggs in any single basket IMO.