Just take the time to consider how you would feel if all the irreplaceable photographs in your computer just disappeared - never to be seen again! Or if the 400+ photographs which you had stored in your camera did likewise.
For many people the words 'back up' are just an American version of 'reverse' when they are driving a car...
To begin with photographs in your camera - many people do keep hundreds in the camera memory - this is really not a good idea for a number of reasons. Specific pictures can be very difficult to find and, when you do find them, they are really so small that they should only be used to show people pictures of your loved ones and other people. Holiday pictures are really a waste on your camera as no-one can really see anything properly and soon lose interest - they only look and comment out of politeness!
You do not have to fill the memory card before downloading your pictures onto your computer - your camera is really just a picture capturing machine - not really a storing/viewing machine. Move them to your computer as soon as you can so that you and others can view them clearly and where they will be safer than in the camera memory... but not really safe until you have backed up your computer files onto some other form of memory.
Your picture files are purely electronic and, as such, can disappear in a flash. You must make sure that you have at least two copies of everything - your main file on your computer for easy reference and a second file, either on a CD or DVD or on an external hard drive which plugs into your computer through a USB port (this is much the preferred method). The cost of these external hard drives has come down significantly and you should really view it as an 'insurance premium' for all your valuable pictures.
When you download the pictures from your camera memory, you should copy them onto your back up external drive first and then onto your computer. In this way you always have a copy of everything and so avoid any chance of having to kick yourself which can be both painful and frustrating!
Computer experts do not talk about 'if' a hard drive will malfunction, they talk about 'when' this will happen - you may be fortunate and never have any problems which would be wonderful - but if you do... it is really worth having the back up!
The feeling of loss when you lose all your photographic memories cannot be imagined - it's just terrible! Do yourself a big favour and back up your files today.