Hey Stephan,
You are an amazing wealth of knowledge…thanks so much! A friend with a 2009 Mac Pro did purchase the GTX 980 and is very happy with it; he had to power from both ports, but is a bit concerned about how hot it might get in summer weather.
I’m leaning towards the same card he bought, but getting the firmware replaced by the guy in LA. I’m HD at the moment, but moving into 4K soon, so this seems to be a good approach.
I’ll also look into upgrading the CPU per your advice…I might be able to handle the DIY stuff to a certain extent. (Mine is, indeed, the Mac Pro 5,1.) Both Premiere and Media Composer will probably run better if I upgrade.
Thanks, again, so much for your help!
Peter