I have a 2010 27" iMac with High Sierra as the most recent OS I can install. It has a Radeon 5750 video card with 1 Gb. I'd like to replace it with a card that supports Metal standards so I can update at least to Mojave and install some supported current apps like Pages and iMovie. Right now, this computer has nothing really useful as it was wiped and a clean install of HS was done. I purchased an Nvidea 765 but the form factor of the card is different than the original card and will not fit the heat sinc. Has anyone changed out a Radeon 5750 with something else that would allow me to install Mojave or Catalina?