What's the best way to automatically backup folders in Mac OS X?

I am getting a external Firewire hard drive for my iMac in the morning. I want to mirror my (Music/Pictures/Documents) folders on the external drive. And only those folders, nothing else.

Also, the external drive is 500 gigs and my internal one is only 160 gigs. The stuff mentioned above will last for a long on the internal one if I can get the movies and TV shows off the internal drive. I can live if I lose the movies. Losing the picture and music would be substantially more painful.

So my question.. What is the best way to automate the back-up of selected folders on a external drive and still be able to use the remaining space for porn?