I am fortunate enough to have a six pac
You can do as many sit-ups as you like but it wont make one bit of difference.
Remember the golden rule young grasshopper....
"ABS ARE MADE IN THE KITCHEN, NOT THE GYM"
Your bodyfat level will need to be approx 10% for them to show through. In order to build on the muscle you already have, you must exercise using mostly compound exercises.
The abs are just like any other muscle and can easily be overtrained by trying to do 1,000's of sit-ups. This will only result in you having skinny thin abs which will not show.
An excessive amount of cardio is not good for building abs either. You really want to stick to the weights if you want a decent pack.
Also remeber that six pacs are also genetic. Everyone's abs look different so you may not be able to achieve a full pac however; sorting your diet out is the first step to success.