Your computer doesn't check at startup if your psu has the right amount of voltage, it doesn't work that way. Even if your computer needed 800W to be able to run effectively, 600W would not cause this problem. It doesn't use all of the power on startup, the idle load is well under 100W. If your psu was the problem, you would hear the motherboard and fans turn on, then you would hear a pop and everything would turn off again.
My guess is that you did not connect the auxilary power cables correctly to your cards and they are refusing to show video. If you were able to have your monitor work straight from your mobo then this would assuredly be the problem. You should post a picture of the inside of your computer, and of the cards. I don't really know what else other than that would cause this exact problem for you otherwise.