24 fps is supposedly the point where one cannot determine that the moving image is really made up of individual images (choppy). TV is at 24 fps, but it uses interlace which cheats a little in that aspect.
That said, even though you don't notice choppiness with 24fps, you can still tell the difference between say 24fps and 30fps, it's a whole lot smoother.
I would start with 24 FPS and if you want smoother go with 30, but never over. That's because people who's computer aren't up to it (flash isn't as effiecient as you would like, it doesn't take that much to slow it down) will have all sorts of problems.
I would say if you have 10 unique objects(same objects, many times don't count) at a time on screen, then you need to be at 24fps or lower. But someone else should comment on this. Plus it's variable because not all objects are created equal.
