Facebook applications are locked to a maximum with of 760 pixels as they are inside of Facebook's own page.
Being an avid lover of the 960 CSS Framework I created this 760 pixel wide grid framework.
For now, the 760.css file is in the SVN trunk. I recommend the reset.css and text.css files from 960, but you can use any you want.
I also committed the PHP based generator I wrote to build the 760.css file, as it might be useful to someone.