The difference is in the processors. Intel on the MBP and PPC on the iMac.
All is not lost though, your files (data that you created yourself like word documents, photos, music, spreadsheets, etc. ) will make the journey reasonably well.
Applications, on the other hand, will not always make the trip successfully. This is because the code is different for an Intel machine than it is for the PPC machine.
Some software is Universal Binary, which means it will run on either machine, but a lot of software is not written exclusively for the Intel processor. (Makes sense as Apple no longer sell any machines with PPC processors.
That said, there is a distinct possibility that a lot of your applications will run on the G5.
Migration assistant will move everything that can run, over to the G5 but does not always get it completely right.
Upgrade the G5 to 10.5.8 first and then try the move. If there is nothing worth saving on the G5, erase the drive and install 10.5.8 and then follow the Migration Assistants directions at first boot up.
I have a MBP 2.16 ghz core2duo leopard 10.5.8, which I use mainly for schoolwork(I'm a teacher), recently I bought an Imac G5(tiger10.4) so I would have more space on my desk, I tried to move files etc. over to the iMac (firewire) and found out that newer software does not transfer to an older version and I ordered an install DVD to put leopard on the iMac, can I use the migration option to get files over to the iMac after the leopard install or do I have to update leopard first to get it at the same level as my MBP?
Thanks for your help