I want to know if you think is ok for you to have a 500x400 active play area on a webgame. the menus are on the top and on the right, the "pages" are switching inside a "screen" that looks like a terminal with soft green lines on the back and a nice graphic frame with height 400 and width 500 (total height of website is 600px total width: 960px) What do you think?

For most webgames I've played, I'd say a 500x500 game area (not including menu's) should be plenty of space. It would say though that a lot depends on what sort of game you're playing, and the control system your going to be using.