I way 115 and am trying to gain weight by adjusting my eating and weight lifting. My personal trainer told me the # of grams of protein I need to eat is 150% my body weight (1.5 x 115 lbs), meaning I need about 170 grams of protein per day! This sounded EXTREMELY high to me. Do any of you know what a good amount of protein is to gain weight? I want several second opinions! Thanks!

Thats about right, especially if you're a hardgainer. Muscle is built from protein, so you need to provide a lot of building material. You need to increase overall calorie consumption and a lot of it will be protein. If you find it hard to get that much in to begin with, start with 1g per lb of body weight. You will staill gain muscle, and probably quickly, but if you are NOT gaining anything with weight training and that level of protein, you'll have to increase it. If you time your meals right, you should be okay with a lower amount to begin with.

The thing I worry about is my waist exploding if I eat too much. I was once 30 lbs heavier and my waist was 8 inches bigger and I looked awful. But I was also not exercising back then. My trainer says that if I weight lift regularly and get aerobic workouts regularly, my waist won't get fatter. Even though I'm light and I have a small waist for a man (about 29 inches), it is not flat and I don't have a six pack. I'm worried that if I eat that much protein, my waist will get worse.

Well, you need to accommodate the additional calories into your diet. Not on top of it.
If you just pile on extra shakes and meats, you probably will gain some fat. Remove a little fat and carbohydrate content to fit in some more protein, but remember that youo need some calorie excess in order to gain muscle as well, so if you gain a little fat while you build muscle, it is normal. With your new lean muscle, it will be easier to burn off.

Thanks; that's really helpful! :)