Go as heavy as you can without pain. And eat, eat, eat! Good clean meals, and suppliment with whey protein, glutamine and creatine. You need to up your caloric intake. How much depends on you. I have a formula in a book at home. I'll try to find it for you. Basically you plug in your weight, activity level, etc and it tells you the approx. numbr of calories you need to maintain yourself now. Then add about 1000 cal per day and you should gain.