I would do crunches instead of sit ups........I would also DECREASE the number of reps.....and Increase the intensity of each one.........really press them out. You shouldn't be able to do over 20-30 in a row properly..if so, add weight. Check out the exercise forum for more info about ab workouts.
a sit up is a full motion from laying down, to having your body up as close to the knees as possible. Crunches, you just target your abs.......maybe lifting your shoulder blades of the ground 2-4 inches.