^This. Ab rollers are great - but most people are too weak to preform them correctly. Why do you want to do weighted crunches? What is your reasoning. Watch Boyle's FSC 3 - he has an excellent core progression in his dvd set that I use with a lot of clients.
Many different core exercises that don't require flexion or rotation.
Half kneel cable press
Half kneel cable pull
Stability ball rollout
Ab roller - After you have mastered the SB rollout.
That's just a few, with no crunching at all. Ever since I took crunches out of my programming a couple of years ago, there has been a lot less back pain at the Studio.