Can anyone answer (or guess) the following: Will Mid 2007 iMac really perform alright with El Cap? I've got a 27" iMac from '07, with 4GB RAM. I upgraded to Yosemite but it was too slow...downgraded to Snow Leopard because it was streamlined and fast. What do you think...will my system really run okay with El Cap, or should I just stick with Snow Leopard?
The first 27" iMac was in 2009. Perhaps you mean the 24" iMac from 2007 which will max out at 6GB of memory. You should consider upgrading no matter which OS you are running.