I am doing 300 crunches, 3 ways (basic front, elbow to knee and reverse) AND cardio for 30 minutes twice a day. I start feeling the burn at 200 or so, but keep going. Am I doing too many? Is there a certain combination that would help me tone faster?

300 crunches? If your goal is to get tone, your doing it wrong; if you want to get rigged abs, your doing it wrong. I don't mean to burn your goals, its good that you want to get tone.

1-5 reps is strength, 6-12 reps is in the in between, 12-15 reps is tone, 15+ is endurance up to a point where it becomes useless.

If you can do a lot already, why don't you add a plate or some weight to your head while doing them? That will encourage your abs to get stronger and more defined.

Do 15 weighted crunches/sit ups for each part of the abs. Add weight accordingly,