Vexel's suggestion...
Here's how...
http://www.apple.com/macosx/upgrade/upgradetonewmac.html
Update: Seems that not all computers have a "Set Up Assistant" in their Applications/Utilities folder. I guess this feature is only available on new(er) computers and Tiger only. I'm using a TiBook and no Set Up Assistant in my Utilities folder.
Transferring files from your old Mac to your new one (taken from Mac Help)
The first time you turned on your new Mac, the Setup Assistant started up and may have asked if you wanted to transfer information from your existing Mac to your new one.
If you did not transfer your settings and files from your old Mac to your new one when you first set up your new Mac, you can use the Setup Assistant to transfer your information.
Locate the Setup Assistant in Applications/Utilities. Open it and follow the onscreen instructions.
You can transfer all of the information from your old Mac to your new one, including network and account settings, files, and even applications. The Setup Assistant will walk you through connecting your Macs using a FireWire cable.
If you don't see the Setup Assistant in the Utilities folder, your computer might not support this.