The figure marked on the power supply is the peak output of it. Normally it will only provide power in around 70% - 80% (If a power supply claims they can provide more than 80% is very good, but it cost a lot .)
because most people just read on the post and buy... they don't ask why and certainly didn't calculate how much pwer they need.
Intel 2 core only need 65W max, Motherboard around 50W Max, nVidia 8 series card 150W, ATI 2xxx series 100W, Ram 3W each, HD 15W each, each USB 2.5W, even if you connect 10 USB, only need 25W DVD 15W,
unless you have 2 graphic card, or many Hard Drive, i think 300 to 350W is plenty enough.