That site which was listed on the previous page is nice, but it isnt accurate at all. I had a 400w antec that wasnt enough for my system, but it told me the 400w was fine. I highly doubt it that it has all the right watages for the video cards. Those things require so much power these days. I agree with mubius, look at the rails.
You also have to consider that your pc uses more power when doing something graphicaly, or running the cpu at 100%. It would be nice if there was some way to monitor the amout of power you are using from your psu.