A couple of weeks in BKK/North; then a couple of weeks island-hopping sounds good to me. Nothing wrong with that plan. In August I think you'll find Samui and the Gulf islands a better bet weather-wise.
KSR really depends on how old you are. It's mostly a younger set there, it's not really tight budget travelers either as many places to stay at KSR are not all that cheap, and you can find far cheaper elsewhere in BKK. It's good for meeting up with the younger set, making friends to travel with, and getting current info. I recommend it as you're on your own.