Flex 2 and ActionScript 3 require Flash Player 9 (no additional library downloads). You're looking at a 98% saturation of the flash player 9.
If you're coming from a programming background, as3 will make more sense to you. As1 was a list of actions you could attach to a keyframe. As2 allowed you to use more complex logic and scripting attached to frames and/or the document.
As3 is a more OO designed language with a faster virtual machine. Also, check out TweenLite, it's really fast.
Flex Game programming:
Flash game programming:
(although I think the MVC is overkill for such a simple game)