Ok, several things, and I think almost everyone has experienced this (well I guess it is safe to say you haven't yet, you are lucky

), but it is NOT a question of IF your hard drive fails, but rather WHEN.
So, how important is your data? If it is full of stuff that is not that important, then no worries. If it is important, then you need to get another drive.
Rule of thumb is that you should have 3 copies of all the important info. That is for photography, anyway. I am sure it is the same for graphic/web design, piece of mind, etc

.
The most important things on my drive is my iTunes library (70GB, just music), pictures, graphic/web design projects. Everything else is just freeware applications, applications I paid for, movies, etc. Stuff I can easily get back.
I have burned a lot of my images to a DVD-DL as well as backed up my iTunes library on 13 DVD+Rs. I am going to be buying a new internal HD, and going to use the one i replace as an external. With Hard drives getting so cheap now, there really is no reason NOT to have extra storage.