Early 2009 iMac 24" 2.93GHz C2D is still going strong. I removed the Optical and replaced that with a Crucial 256Gb SSD plus upgraded the HD to 2TB (created fusion drive), upped the RAM to 8GB also. This should keep me going for another couple of years. I generally will upgrade when I can no longer upgrade to the latest OS which has not happened, yet... Hopefully the next OS upgrade will still be compatible.
Jeff
Jeff