If you have a FireWire disk it's much easier. Or, you can get a FW400 cable and connect the iMac and PowerBook, put the PowerBook in Target Disk Mode (hold T when you turn it on until a FireWire logo appears on screen), and then see if you can access the PowerBook drive on your iMac. You can also install OS X that way if the drive does show up and is good.

And that PowerBook can take a 1GB DIMM in the slot, Apple officially claims it can hold a 512MB but that's not the case. 1.128GB is better than 640MB, trust me.