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Mac OS X Snow Leopard isn't compatible with PowerPC-based Macs, but as you said Mac OS X 10.5, you are referring to Leopard.
First, I need more information about your eMac, so open > About this Mac, and check Processor and Memory. Here are the Leopard requirements > http://support.apple.com/kb/sp517
If you can upgrade, just get Leopard in Amazon or eBay because Apple doesn't sell Leopard, and upgrade. Make a backup of your files
* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
minimum system requirements
* 512MB of memory (I say 1.5GB for PPC at least, 2-3GB minimum for IntelMacs)
* DVD drive for installation
* 9GB of available disk space (I say 30GB at least)
Classic/OS9 Apps no longer supported.
May be available from Apple again???
Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs... ???
There are workarounds if the 867MHz CPU is the only hangup...