hmm weighted crunches help if u find BW crunches to easy.. it will just make them harder and put more resistance.. as for whether or not you should do situps or crunches doesnt really matter if ur gonna do situps us the situp bench and hold a medicine ball behind your head.. might help bit more
lay on the ground with your feet straight out in front of you so that you are flat on the ground. place a DB on your chest, and hold on to it. now, do sit ups, all the while keeping the heels of your feet on the ground.
well, it depends on what your goals are. abviously diet is the key to seeing those abs, and lifting is the key to defining / strengthening the abs. I am more concerned with their strength and how that carries over into my other lifts, so that's why I go more of a weighted route.