Old Socket 775 What's the newest OS I can install?

[bigpoppa206]

it comes down to your CPU only having SSSE3 as later versions of macOS require 4.1/4.2

My specs say the Q9450 supports SSE4.1.

 

[Edhawk]

So what are you waiting for, go ahead and install macOS on your system!

I would recommend you try installing macOS El Capitan or Sierra in the first instance. See if either of those versions work on your system, and then see if they offer you an upgrade to High Sierra.

 

[bigpoppa206]

What? Clover won't work with Catalina? Are you sure?

I use Clover 5119 to boot Catalina on my systems myself. Maybe what is required is to use the Legacy version since the OP system is probably based on legacy BIOS instead of UEFI due to its age.

Did you ever get it running?