Imac 24 late 2008

Your 2008 iMac only supports up to 6 GB of RAM; as it has got two memory slots, it supports a 4 GB and a 2 GB RAM module. You can buy more RAM for your Mac at OWC or Crucial (this does not mean that RAM from other sources will not work).

Crucial does not sell the special 4GB module required to reach a total of 6GB in that model:

http://www.crucial.com/usa/en/compatible-upgrade-for/Apple/imac-%28early-2008%29 -imac8%2C1

Other World Computing (OWC) does:

http://eshop.macsales.com/shop/memory/iMac/Intel_Core_2_Duo_PC2-6400

However, note that the cost per GB of the special 4GB module is roughly four times that of more conventional but smaller modules. Reports here suggest here that trying to add two of the special 4GB modules to a Mac of your generation will result in a failure to boot.

There are experienced users here who are successfuly running 6GB in similar Macs to yours. Our iMac of the same model was going to get the 6GB package but has succombed to some other malady before I could add RAM, so i do not have personal experience with that combo. However, I trust OWC and I trust my friends who contribute here when either say it works.

How much is installed now? I believe that model shipped with 2GB. My son is running 10.11 on a MacBook Pro with 4GB RAM so, unless you plan to use this old iMac for video editing or other stressful tasks, moving from 2GB to 4GB should provide a nice improvement.