I used to keep all my images but have now decided to be more ruthless and only keep the better ones or ones that are significant in some way. I used to have over 100 GB of files but that is coming down.
I do get paranoid about loosing these though so I keep seceral copies. On more than one hard drive and also on DVDs
The main think is not to keep everything on one drive. Hard drives do not live for ever and they will fail at some time. While you can restore operating systems and programs it is the data that you cannot get back if it is not backed up.
I always have at least 2 drives in my PCs and copy the pictures to both as soon as I download then from the card. If you only have one drive then I would copy your pictures onto a CD or better a DVD as soon as possible.
I tend to back up everything to DVDs at about 3 month intervals and then keep the last two back up sets as well. Did I say I wa paranoid
Hard drives a cheap now; at the last PC fair I went to 750 GB drives were going for about £65. They are also very easy to install in desk top machines.