You cannot take the drives out of your G4 and put them in the G5 they have different connections to the computer.
The easy way it to use the migration assistant, start up the new computer and it will ask you if you want to migrate data from the old, click yes and it walks you through the whole thing. It will not copy data from a second internal drive so you will need to copy that yourself.
When switching from a G5 to Intel you cannot simply switch system drives as well the Intel machine will need the Intel version of the OS. Again use the migration assistant when you first start up the computer.