I want to know how much RAM my visitors have and all the information available about their GPU. Is there any way to achieve this via JavaScript or maybe ActionScript (Flash)?
JavaScript, Browser extensions and Plugins are heavily sandboxed that they have limited, to no access to the system for security purposes. Only limited hardware can be accessed directly (with the user's consent), like camera and microphone for JavaScript's getUserMedia or Flash.
The nearest you can get is to have the user install something on the PC that interacts with the browser, like a Java applet. Can You Run It uses this method to check the user's PC if it matches game requirements.