I couldn't begin to guess how many photographs the average photographer accumulates in a year, but as for me, I have a pretty good idea. In 2009 alone, I accumulated over 30,000 images. That's quite a lot and I didn't save every photo I took.
After any shoot, I normally go through my images and weed out the dead-wood. Dead wood being anything I consider a totally wasted shot. Still, even after weeding out the dead-wood, I end up with 30,000 image files. Are all of them worth saving? Probably not, but I always save any usable photograph I was paid to take (weddings, events, things of that nature.) Add to that all of the regular-joe shots I do with the family, next thing you know, your hard drive is getting full.
One simple fact about computers is that eventually your computer will fail. When it fails, the most common failure will be your hard drive. When the hard drive fails, you lose any photographs you have stored on that disk.
Here's my take on backing up images. Do it religiously. You will eventually lose a hard drive and if you aren't backed up, you can possibly lose years of work in a moments time. What are your images worth?
For a few years, I was backing up my image files to DVDs. 4.5 gigs of images is not a lot of images, maybe 300-400 raw files on a DVD. I have a stack of DVD's as tall as the ceiling. What I discovered one day when trying to retrieve some old images from DVD was that a DVD or CD can go bad just sitting on the shelf. For some reason and I don't know exactly what causes it, a DVD has a shelf life. Files I initially thought were safely archived were not. Add to that the cost of a case of DVD's, you end up paying quite a bit for each gigabyte of storage.
I have a new and more simple strategy. I now purchase external hard drives, the cheapest I can find. My current choice are the Iomega Prestige Desktop Hard Drives. These can be ordered online from Amazon or picked up in the electronic stores, and at the moment a 1 Terabyte external HD is running going for less than $100 US. There are other brands out there as well. Look for the best deal.
External HD's are cheap, fast, reliable (more reliable than a CD or DVD) The prices are going to continue to drop over time and the amount of storage they afford is only going to grow.
I buy my external drives from Amazon. Once I fill it with image files, I unplug it from the computer and place a label on it identifying the contents and stick it in the closet. Much more compact than storing DVD's, much cheaper, much more reliable, and lastly, much faster.
So, take it from me. Back up your image files. External hard drives are the best bang for the buck right now.
What are your images worth? They're worth nothing if you don't keep them safe.