thank you phil.
i know what you mean phil but the system power requirement is only one side of the problem. the important thing is my PSU is 650w but i have only 2x 18Amp in my 12V rail. i guess that means 18A+18A = 36Amp = 432watts for CPU and Graphic Card.
my current card uses at least 321w when it's fully loaded and 560 ti uses more than 345w and it could overload my PSU, i just want to make sure is it safe for me to upgrade my graphic into 560 ti or not.