intel i5 2500k,ASUS P8H67-V, asus gtx 560,4gb ddr3..i dont know how to choose a power supply i asked to one of my computer engg said that 550 watts psu of any brand can run this config..is this true coz he is saying to buy a mercury psu which is vey much cheap than corsair and others can i buy that cpu.. and why i have to buy a good quality psu like corsair i thought the amount of watts of psu is the value of psu i thought