I just updated my '07 iMac to El-Capitan

[SystemSettings]

So I just upgraded my '07 iMac and all I can say is it runs great! It has 4 Gigs of ram and a Dual-Core processor, the specs aren't the best but it runs very well for an old computer. It has very little lag and starts up quick and programs open well!

 

[sboychuck]

If you really want to see it come to life, pull out the spinning hard-drive and drop in an SSD with an adapter. I just put a Crucial 525GB MX300 into an old 17-inch MBP and 24-inch iMac, and they have come to life with El Capitan. They will not be as fast as the advertised speeds of the SSD, due to the older SATA spec's, but a lot faster then even a 7200 spinner. A lot less heat as well. This will give them some more years, as long as the displays keep working.

 

[SystemSettings]

Really? Where can I buy one of these?

 

[sboychuck]

Try Amazon and use the AdaptaDrive 2.5 inch. Go to iFixIt and ensure you have the tools and are comfortable doing it. Maybe get an external USB enclosure so you can format it before you install it.