You can definately gain some muscle, but 10-15 lbs is a tall order. Like malkore said, you'll plateau eventually, and it's difficult to say where exactly that'll happen. There are plenty of things that you can vary besides poundage, like angles, range of motion, grips, stances, and speeds and repetition. If you like, see how far it'll get you by mixing these things up. It's always good to have some equipment though.