I would be looking to get at least 40 g of protein per serving, I use the Kaizen pure Whey which is 24 g of protien per soop, I have 3 scoops with each shake as I get much better results from a higher than recommened protien intake, the thing to remeber is that the majority of testing for all the protien & vitamin requirements are using regular people, when you are training with weights & looking to build muscle your bodies requirement for protien is higher, also the more muscle you have the more protien your body needs.
Guy