upgrade iMac G4 to 10.5 via firewire

[familymanmacfan]

Hi, this is my first time posting, and have been looking around for a solution, but have not yet found one specific to my situation..

I have an old iMac G4, 1.25Ghz, 160GB, 1GB of internal RAM (the 2nd slot is not detecting the other 1GB of RAM), and runs on Mac OS 10.4.11.

I want to install 10.5 on the old iMac, and I believe it meets the requirements.
I have an Mac OS 10.5 Leopard upgrade disc, but it is on a dual layer DVD, which the iMac cannot read.

I have a newer macbook (white/intel) with 10.7.5. I hooked it up to my iMac G4 via firewire and using the macbook as an external drive to run the disk. I guess you call it target disk mode?
The iMac detected the DVD disc, displayed it on the desktop, and I double clicked on the icon 'Install Mac OSX'. The OSX Installer window comes up with a button saying 'restart'. I clicked on it, and let it run. The iMac restarted, but when it reboot, it stayed on the blank white/gray apple screen with the grey apple symbol in the middle. No timer, no other activity/progress.. nothing.. It stayed like that, and the disk in my macbook eventually stopped running, and it didn't do anything for a long while. So I did a hard shut down and rebooted it, thought I'd try it again, and the same thing happened.

Am I missing something? Why isn't it doing anything/upgrading? Why does the iMac screen stay blank w/the grey apple logo?
Is there an easier, more correct way to do this? Please let me know.. Thanks so much!!