My 27" iMac has a DisplayPort input to use it as a "passive" screen. Maybe something on the software side like AirDisplay (http://avatron.com/apps/air-display/

My 27" iMac has a DisplayPort input to use it as a "passive" screen. Maybe something on the software side like AirDisplay (http://avatron.com/apps/air-display/ http://avatron.com/apps/air-display/) could be of some use to extend an old iMac lifetime as a screen.