You can t target weight loss..you can do as many sit ups and crunches as you want and yes that will build muscle and tone you if you do enough regularly .But it will not shed the fat. the only way to lose fat in any area is overall body weight loss. Which comes from diet and exercise.
Sit ups they are amazing! Here are some tips..... http://www.shapefit.com/abs-exercises-sit-ups.html
Wat u could do is sit-ups, and some other types of exercise to work around that part..