Don't use AIAF if you have a particular subject to focus upon. With AIAF on, the LCD screen will show 3 "boxes", and the camera will decide which "box(es)" to be chosen and focused upon. However, with AIAF off, you will only see the central "box", and you can use that "box" to decide yourself which subject to be focused on.
AF-assist beam helps the camera to focus. However, if you want to take a shot discreetly without frightening your subject away (eg. taking shots of animals at night), it's better to turn the beam off.
I have an A40 myself. I dont use the AiAF, maybe only in Automode.With it on, it has 3 instead of 1 focusing point. It chooses the nearest and most contrasty object to focus on, which can be fustrating if u want to focus on other things. Turn it off and use the focus lock instead. The AF assist beam helps with the AF in low light. Keep it on, as A40 focuses like crap in anything but bright light. It refuses to focus in low contrast objects in low light. Its a pity, since its such a good camera.