Just put it in a table using the header and the footer files. Place the forum in a cell of the table and set the width accordingly.
You can also use the same technique to place navigation down the side like I did on CaribbeanChoice.com Forums.
And one note about setting a fixed width... make sure you test it at a variety of resolutions. It might not look as expected on some people's monitors. Most people use 800x600 or better, but there are (believe it or not) some people who are still surfing at 640x480! Although that's only 1 or 2% last time I checked. And also take into account that not everyone will have their browser maximized when you are testing.
So don't forget to test. Make sure it looks like you want. Most people hate scrolling sideways.