Re: I'm 155lbs, I want to be 190lbs; read inside.
Nicolas, a good solid weight is where YOU feel good and solid.
6'4" and 175 lbs. does seem a bit on the light side, but you should pay attention to performance and how you feel.
Get a strong grasp on how much you need to eat each day just to maintain your weight. Then, find out how much you need to gain weight (remember to factor in activity in both of these calculations). Slowly begin to add in the calories and keep an eye on performance. Don't worry too much about a number on a scale. If you'r epaying attention to numbers then it should be times and poundages.