Watt Calculator vs Required Watts on video cards

Generally you can run a GPU under it's required spec for power, but not too far. Depending on what the card is, you can just kind of estimate it from experience. I'd say you're fine with any midrange or low level enthusiast card with that kind of tolerance from spec to actual, but not a real enthusiast card. Keep in mind, though, that if you have a 500W PSU, you cannot actually output 500W - the conversion from AC to DC power reduces the overall power you have to play with. The amount that is lost in the conversion depends on your PSU's quality - I've seen it as high as 250W. Higher quality PSU's can get closer to their stated wattage maximum without blowing up.