There is a website that allows you to plug in all your particular computer components and then gives you a calculation of your computer power needs.
http://www.journeysystems.com/?powercalc
Never use a third party website to determine the actual wattage of how much power you need for your computer. Consult your mainboard, CPU, and video card manual for better details. If you do not have a manual try looking online for one, otherwise yes I would suggest using that website as a "general idea". Keep in mind however, not all power supplies are the same and typically do not provide 100% pure power. For example the ThermalTake W0104RU ATX 12V 2.2 Series 650W has an efficiency of 85% what that means is that the power supply can effectively power a computer of up to 552.5 watts PEAK! So when purchasing a power supply always have more effective power than what your computer needs – have to low of a power supply can result in damages. For more information on computer power supplies see wiki article.
References:
ThermalTake Power Supplies