Upgrading old iMac G5 PPC from OSX Tiger to Leopard

Hi. I was given an old iMac PPC (G5, 1.8Ghz, 1GB RAM) from a friend. It was in such a state when I received it that I decided to wipe the machine and start over. I now have OSX Tiger running on it beautifully.

My friend also gave me some official Apple DVDs for OSX Leopard. However, I have now realised that they are the "wrong" discs despite being genuine, as they were intended for a modern Macbook Air, and I have learned the hard way about how Apple makes DVD sets specific to the hardware they're being installed on.

I have the grey Leopard install DVD set, and apparently (from my research) I need the black ones ... ?!

Can anyone here advise me on how to proceed? Where can I source these black install/upgrade DVDs (or CDs)? Once I have them, are there any potential issues I need to watch for? I have read horror stories about upgrading to Leopard and various things not working properly, like Sleep Mode and USB ports.

I cannot seem to connect Tiger to the internet currently as I don't understand the complex old-school setup process, i.e. I don't seem to be able to just "plug and play" with the internet, which is one reason (as well as features, security) I'd like to upgrade to Leopard. It's such a lovely little machine otherwise. Also, I am worried that doing an internet upgrade to Leopard will be slow and produce more problems (again, I have read horror stories about online installations crashing horribly and resetting).

Anyway, please help or advise as you see fit. Thanks in advance.