I am also trying to gain lean mass, while shedding some extra fat. I therefore can't set a weight goal myself either!
I have gained about 25 lb over the last year or so and I would say that you would find it very difficult if not imbossible to gain that much weight by using p90x. What you need to do (along with a good diet) is get into some powerlifting and bodybuilding exercises that will increase your free testosterone and mass), having said that, p90x is an OK place to start at for the first month or two into your building program.
As 01gt4.6 says, you will need to eat well. Being a skinny guy means you will need to put in a lot more calories than you are used to.
As far as protein goes, it is standard to take in at least 1gram protein per lb of body weight each day.