The purpose of adding whey protein is simply to increase the amount of high quality protein in your diet. How much to use or whether it will be beneficial depends on the rest of your diet. Meat eater? How much meat-fish-fowl/day? Remember that each gram of whey protein has about 5 calories in it, so if you add calories in one place, you have to take them away from the rest of your diet.

Here's one view:
http://www.powersupplements.com/protein.htm
I'm not endorsing any particular whey protein product.