Actually that GPU has a requirement of no less than 450 Recommended 600 or higher. That and your processor and the 1600MHz RAM you put in, 400Watts just is not covering it! That is my prediction. my build just happened on friday. my specs are below. I had a 500W PSU sitting here but bought a 650 instead.
the 400w cx is fine, i tried the same with a different motherboard and instead of a phenom 955 is was a phenom x2 555, and it ran fine, and the power was about the same.