The weighted ab machines at the gym are great. You get more reps done as you are in a more comfortable sitting possition.

Fat is fat, no matter where it is on our bodies. If you want your belly to be smaller and the muscles to be more defined you first have to lose that fat that is there. Aerobic exercise combined with ab exercises will do the trick. But the cardio exercise is more important. If you search ab exercises on the web you'll get tons of results. I do 5 or 6 different exercises a day and change them up all the time, that way I'm targeting all areas of my abs. (Don't forget to work your lower back as well)