Just take an ethernet cable, like coffee said, make sure your two computers have file sharing open, and have one access the other (all this is done in System Preferences under Sharing). I just did this about a month ago (iMac G5 to a MacBook).
But, before you delete anything on the G4, remember to write down all of your software registration numbers (assuming you own them legally). Some software will require you to re-input this when you fire it up on the new machine. Not all programs, but just in case ... it's a good idea.
And just to chime in to Del's question: It's nice. If you use it for portable word processing at a game, I easily get four hours off a full charge. It does have a lot more bells and whistles than the earlier versions, even on the basic model. But I hardly use them. Just a drain on battery life. Most of all, I'll be able to have Windows XP on a separate part of the hard disk, so I could go to any paper and work whether they have Windows or OS X.