If the sit ups kill your back than you are doing them wrong. The muscles of the abdominals must be consciously used (that means don't use gravity to pull you).
As for Jammer: There are many different kinds of crunches that will work different sections of the abdominals so you can't get great off of just one exercise (hence the reason why sit ups are not useless).
I will explain some of them but not in full detail because I don't want to take the time to do that.
1) Crunch: This only strengthens the rectus abdominis (upper abs). Sorry but you can't get the shape you want just off of this. You lie on the ground with your hands placed on your ears (or you can place hands crossed on shoulders) and lift your legs off the ground with your femor at a 90 degree angle (you don't HAVE to have the shins parallel to the ground). Keep your head lifted for less back injury (personally I make sure my shoulder blades don't touch the ground) and then let her rip! Begin crunching.
2) Crunch Twist: For the serratus anterior and outer abs. Its like a normal crunch but you don't raise your legs and you twist (reach for your knees). This is similar to the "bicycle." You should raise your head or keep your shoulder blades off the ground to prevent back injury.
3) Standing Oblique Crunch: It works... the obliques. Grab two dumbbells and lower one side as far you can (but not so far that your spine hurts). Than raise that side and perform the same to the other side. I don't fully know how to explain this so I hope it helps. You can try Side Crunches if you like.
4) Butterfly Crunch: Another variation of the crunch that strengthens the lower portion of the abs. Follow the same rules for the crunch but keep your legs on the ground instead and form a diamond shape with the soles of your feet touching to complete the diamond.
Thats my post for the day.