your post does not say anyting about your system. in order to determine if 400 watts is enough, we would have to have been told some things like...processor, number of hard drives, video card make/model, sticks of ram and type. any other things you ave plugged in to the beastie.
at first guess i would say that the PSU sounds like the problem. have you tried unp-lugging the fan you installed to see if your FPS returns to what it was before?