thiduck :
So, I have a 800w psu but my load is only as said 600w ,so I am consuming 600w only?? Just to be clear.(so no matter what you have, it is always better to have more powerful ones, for future proofing??)
Not exactly.
True, a psu will conume only the wattage that is demanded of it, regardless of it's maximum capability.
Also true that overprovisioning has some benefits:
The psu fan will loaf and be quiet.
You will be prepared for some future upgrades, mainly in graphics cards which take the bulk of power demands.
But, realize that newer graphics cards are becoming more power efficient. Today, even a GTX690 only needs a 620W psu.
But some drawbacks:
A vastly stronger psu will be more expensive.
A PSU operates most efficiently in the middle third of it's range; that is ideally where you want to be.
A overly strong psu will come with more power leads that you can use, a cable management issue.
Whatever you do, buy only quality.
Here is one guide to psu quality:
http://www.eggxpert.com/forums/thread/323050.aspx