This screencast shows how to use the Variable Grid system to create a grid with the widths and number of columns of your choosing. There are a number of tools out there that help with this but the cool thing about this one is that it writes a CSS file for you that uses the exact same classes as the 960.gs grid system. That way you can use everything your learned about the .container and .grid_x and .alpha and .omega classes with the 960 grid system but using your own grid dimensions.