Its not just about Wattage, but rather quality of PSU which is imp when CF is concerned. Sadly, yours is not reliable, even poor, for high loads and not at all recommended for CF of any 2 GPUs. 400W max wattage doesn't mean your PSU will run it. CX are ok to run only if load is less than 70% all the time, or they fail or even explode. Get a good 650W or above PSU, like the EVGA SuperNova NEX 650W. Any 650W+ PSU from Antec/ EVGA/ SeaSonic/ XFX will be good to go