Either way, you need to decide if you want to save a bit of money on 10.5 or get iWork and iLife as part of the package. The latest Tiger-supported version of Transmission, as well as a Panther supported version can be found. My current Mac came with 10.5 so the upgrade to 10.6 was cheap. Transmission is free to download and use on any machine running Mac OS X 10.4.11 or later. I was a ludite happy with OS 9 on a Wee Pismo. 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.