Last night, I upgraded my wife's G5 iMac (running Tiger 10.4) to a Intel iMac (running Leopard 10.5) using Migration assistant (on 10.5) to move all of the data. Some thoughts if you are doing the same:
0. Set aside a few hours. Although Firewire can theoretically transfer 180 GB per hour; for me, the migration process took 2 hours for 40 GB. This might have been because I was using a 15 foot cable [the max cable length] - the only one I could find. Get a six footer if you can.
1. Make sure you have a Firewire cable first; you need a 6-pin to 9-pin cable (warning - they are currently in short supply) - and if you can, use something shorter than 15 feet [the max length allowed]). This is because the G5 only uses Firewire 400 (6-pin) and the Intel only uses Firewire 800 (9-pin). Why Apple couldn't put a additional Firewire 400 port in the Intel escapes me.
2. During setup on the Intel iMac, it will ask you if you want to migrate data now or later (using Migration Assistant). Better to do it now. Follow the instructions, except ...
3. It will get to a screen where it is trying to move the Applications Folder - it will take MUCH longer than it thinks it will take - be patient - those last few minutes will take an hour.
4. After you are all done, you will probably have application problems:
5. If you try to fire up Mail and it does nothing, follow the instructions here: http://discussions.apple.com/thread.jspa?threadID=1849306&tstart=30. They worked great with one exception - before starting, do a force-quit on Mail to stop it (under the Apple menu).
6. To make sure that other applications will not have problems, reinstall all of them from the original disks (or download images). Take this opportunity to visit the applications' web sites to see of they have newer versions - you may be using a PPC version (for the G5) and a newer Intel version may be available (OpenOffice is one).