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The latest OS X version for PowerPC-based Macs is 10.5.8, so you can't install Mac OS X Snow Leopard.
Instead, check that your Mac can run Mac OS X Leopard > http://support.apple.com/kb/sp517 If so, you can buy Mac OS X Leopard at Amazon.
Then, make a backup of your Mac with Carbon Copy Cloner (there's a version for Tiger) and finally, insert the Leopard DVD and upgrade. After upgrading, open > Software Update, and install the most recent Leopard version.
After upgrading to Mac OS X Leopard, you won't be able to use Classic applications