Mac OS X 10.5.8 upgrades

Back up your data, click here, and read the entire page. Mac OS X 10.7 and newer don't support PowerPC software such as Microsoft Office 2004.

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To followup, Niel's caution, this tip addresses what PowerPC software means and why it is important:

http://discussions.apple.com/docs/DOC-6271

Upgrade to Snow Leopard, then update to 10.6.8. Snow Leopard Purchase

Check that your computer is compatible with Mountain Lion/Mavericks.

To check the model number hold down the option/alt key, go to the Apple menu and select System Information.

• MacBook (Late 2008 Aluminum, or Early 2009 or newer) model number 5,1 or higher

If your computer isn’t compatible, you might be able to upgrade to Lion.

Computer Compatibility - Lion Lion