Working on stomach depends on what you need to work on. If your guts hang out, then you neet to strengthen the muscles. But if it's trying to get definition, cardio's the best way to do it. Just look at runners.
As a suggestion for your ab workout for general muscle tone, do crunches instead. That'll hit upper abs. Throw in some leg lifts for lower abs. If you break the ab workout with the pushups, you can do more of each, throw in a couple of other exercises in between and you got yourself another small cardio workout. ie: i'll do 10 pushups, 15 situps, 10 leglifts, 10 modified dips, 15 squats, 10 calf raises. All with small breaks in between, and i'll do the cycle a few times. Get's the heartrate going which is always good.