Q: How to update OS X 10.5.8 to he latest El Capitan?

It should say something like Model identifier: iMac x,x

OSX 10.5.7 may not have that info displayed.

Open the Terminal from the Utilities folder and copy and paste this in, then hit return.

sysctl hw.model

From what I have read, I think the following depicts your Mac. To check, depress alt/option and at the same time click on the Apple symbol (top left in the Menu bar), select System Information and read the Model Identifier.

It will handle the latest release of OS X

Your Memory could be upgraded to 6 GB consisting of 1x 2GB and 1x4GB RAM module. Ask if you need the specific specification.

The iMac could also handle an SSD at SATA II speeds which will make a world of difference in your Mac, giving you another couple of years good service. Ask if you need further instructions

H

HTH

Leo

To DB,

Apologies, only saw after posting that you were heading in this direction

Leo

no problem and no need.

You're welcome. You definitely need more memory. 4GB is really the minimum you should have for El Capitan. I was asking about which model/year you have to be sure you could upgrade at all. If you think Leo is correct and your iMac is an iMac7,1/Mid 2007 you'll be fine. You will need to purchase Snow Leopard from Apple and upgrade to that first.

Glad to have been able to help.

Have fun with your Mac

Leo

The 2 places I’ve seen recommended most to buy reliable RAM are below. I have purchased RAM several times from Other World Computing and have always been very satisfied with the product and service. They have on-line instructions on how to replace the RAM. OWC has also tested RAM above what Apple states is the maximum. I now have 6GB installed on a early 2008 iMac supposedly limited to 4 GB and noticed an improvement.

Crucial

Other World Computing 

Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.      

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.