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
Power supplies are one area you do not want to go cheap. When a power supply fails it can take all your other components with it. Cheap power supplies that are rated for a set wattage does not mean that is the constant rating but can rather be a peak rating.
Power supplies degrade with use and age. I typically will go 100+ higher then what I need to help insure that it will continue to deliver adequate power for years to come.
I'm not sure how your question relates to the topic you posted.
You definitely don't want to go cheap with a power supply, buying a good brand like corsair/antec/seasonic is always the best bet, a cheaper power supply simply wont do what is rated to. Shaw have a 1500w power supply i wouldn't trust to charge my phone. Its not just about how many watts, its about quality of components.
If a 550w psu is what your after look at one from a decent brand, might cost a little more then a cheaper brand one, but it's going to last longer and not blow up on you and possibly fry parts of your computer. I'm not certain the exact wattage you need for a gtx 560 though there's usually some indication in the manual that comes with and whatever it says minimum i would add 100+ to it like baralis said.