Q: How do I upgrade my iMac8,1 OSX 10.5.8 to the newest version?

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; some other installed software may also be incompatible.

Going from 10.5.8 to any newer Mac OS X version requires a disk or access to a Mac running Mac OS X Server.

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Here is what you need to do for your model:

A. Upgrading to Snow Leopard

You can purchase Snow Leopard through the Apple Store: Mac OS X 10.6 Snow Leopard - Apple Store (U.S.). The price is $19.99 plus tax. You will be sent physical media by mail after placing your order.

After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store. Access to the App Store enables you to download Mavericks if your computer meets the requirements.

     Snow Leopard Minimal Requirements

       1. Mac computer with an Intel processor

       2. 2GB of memory

       3. 5GB of available disk space

       4. DVD drive for installation

       5. Some features require a compatible Internet service provider;

           fees may apply.

       6. Some features require Apple’s iCloud services; fees and

           terms apply.

B. Upgrading to El Capitan

You can upgrade to El Capitan from Lion or directly from Snow Leopard. El Capitan can be downloaded from the Mac App Store for FREE.

To upgrade to El Capitan you must have Snow Leopard 10.6.8 or Lion installed. A minimum of 2GBs of RAM. At least 20GBs of free space for the installation. Download El Capitan from the App Store. Sign in using your Apple ID. El Capitan is free. The file is quite large, over 5 GBs, so allow some time to download. It would be preferable to use Ethernet because it is nearly four times faster than wireless.

      Macs that can be upgraded to OS X El Capitan

         1. iMac (Mid 2007 or newer) - Model Identifier 7,1 or later

         2. MacBook (Late 2008 Aluminum, or Early 2009 or newer) - Model Identifier 5,1 or later

         3. MacBook Pro (Mid/Late 2007 or newer) - Model Identifier 3,1 or later

         4. MacBook Air (Late 2008 or newer) - Model Identifier 2,1 or later

         5. Mac mini (Early 2009 or newer) - Model Identifier 3,1 or later

         6. Mac Pro (Early 2008 or newer) - Model Identifier 3,1 or later

         7. Xserve (Early 2009) - Model Identifier 3,1 or later

To find the model identifier open System Profiler in the Utilities folder. It's displayed in the panel on the right.

     Are my applications compatible?

         See App Compatibility Table - RoaringApps

Note when Apple says "minimum" RAM they really mean it. The computer will barely run with 2 GB and El Capitan.  Many people say you need 4 GB to make use of the computer and preferably 6 or 8 GB RAM.

The maximum RAM for an iMac8,1 is 6GB.

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If you have access to another system, you might be able to directly use the downloaded El Capitan installer to install onto the iMac8,1 in Target Disk mode, or use DiskMaker X ( http://diskmakerx.com/ ) to build a bootable drive from OS X installer program (the one you download from the App Store) and then use that to install onto the iMac8,1, either over the current OS or by erasing the iMac8.1 (after backing it up of course!) and installing it onto the empty system.