Sit ups work the lower abs <---
Crunches work the upper abs <---
Leg raises work the lower abs and obliques if done in an angle like mentioned above.
The first leg raise shown in the above post will work the obliques and lower abs pretty darn good ( I like those exercises btw

).
The next one will work lower and upper abs.
FOr your workouts I would really consider doing the whole body.
Do you do your triceps and chest?
'cause if you only do curls and neglect your triceps you will get unpleasant imbalances.
Anywho. Diet is how you will get abs. You can do one million crunches/sit-ups per day but if you don't eat properly your abs will never show.
Concentrate on the whole, not just the abs.
My suggestion:
-Whole body workouts.
-Proper calorie deficit (eat less than you need to maintain weight).
-Persistence.
Good luck!
Eric