What PSU is this? If it's not a good brand (e.g Corsair, OCZ, Be quiet!, Antec, Seasonic, etc.) then you risk damaging your hardware.
1200W should be enough at stock settings.
It's Chieftec. When overclocked, a single 6990 can consume up to 450w, right? In this case, I will need a 1,500w PSU. Or maybe I'm wrong?