Make sure you are aware of the system requirements for Keynote and the rest of the iWork programs before you buy a retail box. That's one of the reasons why you should download a trial version of the product.
The current version, Keynote 08 (or Keynote 4) runs on Mac OS X 10.4 Tiger and 10.5 Leopard only (and later versions of OS X). And if you are running an earlier version of OS X, you'll have to find an earlier version of Keynote and iWork as well (or upgrade your OS X version).
You can find older versions of Keynote and OS X on auction sites like eBay but be aware that the newest version of Keynote always has more features. Also, you might not be able to open Keynote presentations created in the new version inside older versions of Keynote, although the opposite is not true.
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