According to Mactracker, your 20" Early 2008 iMac maxes out at OS X 10.11 El Cap.
You can d/l El Cap by simply clicking on the link on this page, although if the iMac is running OS X 10.7 Lion or later, I think you can do the entire job, including erasing its HDD/SSD in Recovery mode.
Hope that helps.
The new Great Equalizer is the SEND button.
In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
