The Setup Assistant, included with computers that have Mac OS X 10.3.4 preinstalled, can transfer data (including account settings and home folders) from a previous Mac (that's using Mac OS X 10.1 or later) to your newer computer. You'll need a FireWire cable to connect the two computers together. Setup Assistant offers to transfer (or "migrate") data from your older computer when you turn on your new computer for the first time. Make sure the older computer's firmware is up-to-date first.
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You hook the old Mac up to the new one using a firewire cable..
Tell me about the Setup Assistant's "Tranfer Data" Feature
The Setup Assistant, included with computers that have Mac OS X 10.3.4 preinstalled, can transfer data (including account settings and home folders) from a previous Mac (that's using Mac OS X 10.1 or later) to your newer computer. You'll need a FireWire cable to connect the two computers together. Setup Assistant offers to transfer (or "migrate") data from your older computer when you turn on your new computer for the first time. Make sure the older computer's firmware is up-to-date first.
there's a lot more info at the site