It entirely depends on your school's grading system. Mine does 90+ is 4, 80-89 is 3, and so on, then 1 point extra for pre AP and 2 points extra for AP classes. Some do a +/- system, where it gets more complicated.
I would say if you are on a 4.0 scale, where 4 is the highest possible grade, then aim for a 3.5 or higher to get in to a good college. If its like mine, where in theory you could get a 5. something by taking AP classes, aim for a 4-4.5 I'd say.
But it all depends on how your grading system is, and how many AP classes your taking. For example, if you get an 85 in a pre AP class, this would get you 4 points. If you got a 95 in a level class, you would also get 4 points, but the AP class would look better on your transcript. So even if you sacrifice a whole letter grade in a pre AP class, or 2 letter grades in an AP class, it would look better than an A in a level class, and give you the same amount of points, since you most likely worked harder to earn that grade.