Seems that PSU would be fine if it was powering only the GPU. But remember you have other parts; CPU, HDD, DVD, Motherboard and fans. Each needs some power, and if I remember from what Skuzzy has said, you need overhead for startup. If it needs 400watts, and the PSU has 400watts, what about the Extra wattage needed for the rest? That's, what, 50-100 Watts?
I may be wrong about this though, I am not very informed about the whole Overhead thing. I'll let Skuzzy or someone explain better.