G5 won't use startup disc with OSX 10.5

I have a G5 1.8 dual core, 4.5gb of ram running OS X 10.4.11. I battled to install 10.5 on it and ended up taking the secondary drive out and sticking it in my Mac Pro. Eventually managed to format it 'GUID Partition' and got the OS installed. Stuck it back in the G5 and it appears as a startup disk option. But when I select it the machine keeps reverting back to the original 10.4.11. If I disconnect the drive with 10.4.11 I just get the little alternating face and question mark as if I have no drive in the machine. Also tried swapping the drives around the slots as a last ditch attempt that I thought wouldn't work and I was right. Can anyone help? I fired up this machine and stuck a third party airport card in it but in the three years since I used it in anger a lot has changed. I can't get the latest iTunes or Adobe flash played installed and a whole bunch of stuff needs 10.5 or above. Any ideas anyone. I am going to try disconnecting the airport card and I'll come back if it makes a difference.
Thanks
Phil.

Thanks

yes I do mean dual processor, not dual core, oops. I do have the discs. When I tried direct from disc it simply told me I couldn't do it on this PC. I then tried in through fire wire and it wanted me to partition it GUID. Pulled the drive from the G5 and stuck it in my Pro. Same thing, wanted me to format GUID. I didn't get the option to format GUID until I erased the drive in the Pro. Having formatted it as requested by the machine I managed to load 10.5 but now the drive is back in the G5 it won't boot from it. Is this because of the partition format? Not at the machine right now but if I need Guid to install from the pro but Apple map to run on the G5 I am stuffed as it simply won't load from disk on the G5.

Any more thoughts gratefully received.

Thanks.

Thanks again

So I need to format Apple Partition Map and install from a black retail disc. I'll be back with either more thanks or more questions. Cheers people.

Leopard Disc

Jesus H Christ, why are Leopard retail discs so expensive? My Snow Leopard cast me about £25 and this earlier version is £60 at the cheapest and that's a scruffy copy in a paper sleeve.