I just got back my mid 2007 20" iMac from my son because it was to slow for him. I am doing a clean install of el captain as I type. I was thinking if I could use the display as an extended display from my late 2015 27", not sure if that is possible. Mini Dvi 2007 to TB2 2015? I then thought I could try and make it a faster machine. 6GB Ram, 256GB SSD, maybe some other cheap upgrades, not sure if I could change the CPU or if anything else would make a big difference. Please let me know your opinions.