So, I got a new Sapphire 7950 Vapor-X today. I've installed the card into the PCI-e 3x16 slot, attached two 2+6 power connectors. There's no display on the monitor on booting up, the gpu lights up and the fans do move on switching on the power button.
The onboard graphics adapter is working.
Motherboard is Asus P8Z77-MPRO. PSU is Corsair TX650 V2