desktop pc

Hello I've recently installed macOS Catalina on an LGA 775 system with an Intel Core 2 Quad Q8400, everything works fine but I wanted to upgrade to MacOS Big Sur, I've made a first attempt to boot with OpenCore but I get an error "Boot mismatch" and nothing after, so I've tried with clover, and...

Can someone advice me which OS X I should install on this old desktop? Any help would be greatly appreciated.