It’s very easy. When you start up your new iMac or MacBook, you’ll have a chance to plug in your older Mac with either a FireWire cable or an ethernet cable and use the Migration Assistant to move files and settings over to your new Mac. Migration Assistant will move your OS X home directory and your programs and settings.
This will include options for moving email settings and your email itself. Email file formats have changed over time with changes in OS X, and if you’re moving from 10.4 or earlier, your emails will be converted to the newer format used in 10.5 Leopard.
If both your new and old Macs have FireWire, you can start up your old Mac in what’s called Target Disk Mode, and your files will be transferred over the FireWire cable. Or, you can use an Ethernet cable, too, to transfer via a network, if you get a new MacBook that doesn’t have FireWire, or your older Mac doesn’t have FireWire. The Migration Assistant will give you a choice.
You can get cables at Best Buy or Staples or Radio Shack for $10 or less. If you use ethernet, you don’t need what’s called a “crossover” cable, one that is wired especially for computer-to-computer networks; you just need a plain ethernet cable two or three feet long. If you use a FireWire cable, either a 400 or 800 Meg speed cable will work.
Either way, the Migration Assistant will walk you through the transfer.
Afterward, you may need to reauthorize some programs – such as Adobe Creative Suite – on your new Mac, so find your registration information for them. Some older programs (depending on how old your “old” Mac is) won’t run under the newer Intel processors and can’t run because they don’t have the OS 9 emulator that they ran under on older Macs and Macs that used the PowerPC architecture.
Some older programs will run under 10.5, but it might be best to go ahead and change. As an example: if you have used Appleworks and are moving to Pages, you’ll need to drag and drop and translates your Appleworks files into Pages. Appleworks will run under 10.5, but it’s probably best to go ahead and change to Pages.
There’s more at Apple on How to use FireWire Target Disk Mode. You may need to install a software update on your older Mac called the DVD and CD Sharing Setup, which can be downloaded here in order to be able to transfer over ethnernet. There’s a FAQ on using Migration Assistant here at Apple.