The 750 Watt means the maximum that the power supply can deliver. It wouldn't be good if it was delivering that constantly. Usually there's a peak when starting the computer but after that the power consumption goes down. If you want to know how much your PC is actually using you can use a power calculator like http://extreme.outervision.com/psucalculatorlite.jsp to estimate the maximum consumption of your components, but the most accurate is to use a power load meter to measure the actual use. It is most likely quite a bit lower than 750 Watt.