These are the items involved:
- CPU - tried
- RAM - tried
- Motherboard - tried
- PSU - tried
- GFX card
- Cable
- Monitor
Is it worth trying a different cable and/or different monitor? Possibly even a different type of connection - DVI instead of HDMI? VGA even?
Normally when you have one of those cards installed, the motherboard disables the onboard graphics. I think there's a setting for whether to initialise the PCI-E graphics first or the onboard first. Worth checking that.
Also worth seeing (ruling out) whether it's a legacy/UEFI bios issue. I'm sure I read that some cards don't have UEFI bios. Worth turning Fast Boot and Secure Boot off and try turning legacy boot and option bios options on. It's a stab in the dark....but nothing to lose.