Apple G5 1.8 OS upgrade

Hello to all,

I'm a newbie to Mac (don't know why it's taken me so long - but that's a discussion for another forum)...

I recently was given an iMac G5 1.8 (non-Intel based processor) by a friend who got it from someone else. The original owner upgraded the RAM memory (to a max of 2GB) and also upgraded the hard drive to 320GB. Problem is that the hard drive has been wiped clean and doesn't have any OS on it. Furthermore, I don't have any installation discs for the machine either [ :-( ].

I want to install the highest OS on this machine that I can which, according to several sources on the internet as wall as Apple tech support, would be Leopard (10.5.8). The machine originally shipped with 10.4. Lastly, this machine also has a separate built-in graphics card.

So here's my question(s): Do I need to install 10.4 FIRST before attempting to upgrade to 10.5.8 and, if so, do I have to buy installatin discs that are/were specific to this model? Or can I just go ahead and buy some 10.5 recover discs and start there? I have heard conflicting stories and being new to Mac's, I'm not sure how to proceed (pc's are my background...but don't hate me for it).

Understandably, I'd prefer to start this process by installing 10.5 from the start. Since I can't buy any of these discs from Apple, I'm left to search sources like eBay for this software. On eBay I see LOTS of 10.5.X software discs that look like they were probably originally included with another Mac machine. If I can start by installing 10.5 directly onto my machine, does it matter which of these disc sets I purchase? I've also read on the internet that it was HIGHLY recommended that I purchase the RETAIL version of 10.5 (and ONLY the retail version) if I'm going to take this direct approach. Furthermore, I also read that, for some machines that have independent graphics cards (like my machine) I may have to first disconnet the internal graphics card before attempting to install 10.5.

I'm obviously very confused and feel like I'm totally out of my realm here - so any advice would and will be greatly appreciated. PLEASE, when responding, keep in mind that I'm a non-Mac guy that is just coming into the fold (so to speak) so please take that into consideration when giving me advise...

Thanks in advance!

Mark