That thing is fairly inaccurate. I know for a fact that my system wont boot with less than 500 watts yet it shows me only needing 450. Although I guess it really depends to a degree. Still, it use it only as a rough guide, even if its more reliable than manufacturer stated wattage needed.
Your system won't boot with less than 500 watts?

I would be quite surprised if there's any non-overclocked desktop that doesn't boot with a quality 500w PSU.
At full load, a skulltrail system with dual QX9775 processors uses well under 500 watts. (1, 2, 3)