Well my friend asked me this question that he had specs.:
motherboard -gigabyte g 31
processor-intel core 2due 2.8 E7400
ram - 4gb DDR2
GPU-Sapphie readon hd 7730
my psu was with casing it has 450 wat
he told that when a game starts he have 55-60 but when a mission stats it goes to 15-20 I think...