generally yes.
It's not an exact number, but the GPU doesn't know what else you have in your computer, so it over estimates a safe number.
It's actually "If you have 500watts it will work" When you're only using 350watts in reality.
Also, alot of PSUs are made really poorly, and don't provide the watts they actually say they do, so you may have a "400 watt" psu but it can only provide 300 or so.