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In theory, your Mac should be compatible with Mac OS X Leopard. See > http://support.apple.com/kb/sp517
If the DVD came with a Mac different than yours, you can't use it to upgrade your Mac to Mac OS X Leopard. However, if you bought it at a shop, you can use it. Make a backup of your files, insert the DVD and upgrade. Note that Mac OS X Leopard is the most recent version for PowerPC-based Macs, so you can't upgrade further.
If you have to buy Leopard, you can find it at eBay or Amazon. Apple doesn't sell it anymore because it's no longer supported for updates