There's no maximise key or action in OS X. This truly sucks - if only you could change the double-click on the title bar to maximise. Argh! That's one thing that Windows got right.
So as a work-around (and this takes some highly pleasurable fiddling), you can assign a key to maximise the current window. See this article from the macosxhints site.
Part of this work-around is to install Quicksilver (link: http://quicksilver.blacktree.com/).
Quicksilver is an application launcher with full application integration and scripting capabilities. This is what allows you to bind the AppleScript command to a keystroke combination.
Its UI is polished and slick, with even the most mundane tasks like loading add-ins getting a spinning-cube effect. Nice.
Highly recommended - this joins Automator and AppleScript as a set of useful tools you can use to get results from without reading any documentation.