Dave,
You are correct the camera does overwrite data when it is full, starting with first record. It does not take pictures as such. It records 20 seconds at a time and will start-stop again so you will get time stamps like 073020413.AVI, 0730400413.Avi, etc.
I forgot to mention the four audio settings too. "You are trespassing!", "You are being records by a security camera", dog barking and OFF. I have heard you can record your own sounds in one model, but not sure about this one SmartGuard.com has more info, sorta.
Steve