Tiger (10.4) would be the best Mac OS for your machine. If the machine is full ram and has a fast drive you can also install Leopard (10.5) but the machine performance could be a little sluggish. Now your problem is the installation disk. You'll have to find a retail installation disk (black disk) cause your iMac disk (grey disk) won't work with the PowerBook. This is a specific installation disk for the iMac only. Also, the hard drive must be formated with a mac, not with a PC.