Most shutter lag can be attributed to auto focus.
The manufacturers are starting to address the problem with the newer cameras.
A newly introduced camera by Olympus is right in the middle of your price range and has a total shutter lag of 3/10ths (0.3) of a second. The lag is 0.1 second if you prefocus by pressing the shutter button half-way down. That is fast by digital camera standards.
Take a look at the Olympus C-7000 at:
I have Canon Powershot A20, I want to upgrade it, the major concern at this time is the "shooting time", sometimes, I can point and shoot the picture quickly, but sometimes, it needs considerably a long time(when take a picture of a camera shy girl, most likely, I won't be success), I guess it is because the len was auto adjusting. (I understand if it is in the low light shoot mode, the shot gate need more time to operate).
so what are the suggestion for a "quick" digital camera?( from 400 - 600$ range).thank