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The most recent version is OS X Mountain Lion and it's compatible with your computer, so it's better to upgrade to this version.
Open the App Store and purchase OS X Mountain Lion. While it's being downloaded, make a backup of your data with Time Machine or Carbon Copy Cloner, and check that your applications are compatible > http://www.roaringapps.com
When the download finishes, the OS X installer will appear, so follow the steps. After upgrading Mac OS X, you will have OS X 10.8.4, so open the App Store and download Microsoft SkyDrive