I saved $$$ when I bought retail 10.3 and 10.4 disks. I see someone is selling gray disks from a iMac! Buy that. The ones advertising as 'Retail' tend to run on the higher end. Just scanning eBay you will find 10.5 running from ~40 to ~200. You are essentially paying ~140 for 10.5 plus the added goodies of iLife and iWork. If you want to save $ you may be able to get 10.5 from a dealer-RETAIL DISK-say on ebay, but if you poke around HERE you will find that often results in people buying machine-specific DVDs which become glorified coasters. Apparently, you can but it is more expensive.
Salsoulkid I did not think you could unless you already had 10.5. According to my Mactracker, the highest OS is 10.5.8.
