Just look at all the best sites in the world, such as http://www.bbc.co.uk
and you will see most stick to a fixed width.
It takes a lot of effort to layout a complex page well if the width is constantly changing. It's not a problem with text only pages like forums but when you start including images and other graphic elements things become more complex.
Take a look at the banner on http://www.windsurf.me.uk
and you will see that this is made up of three elements to make it extendable. The centre element gets obscured when at 800 but is gradually revealed as the width increases. Above 1600 wide gaps apear between the three images hence me probably opting for a fixed width in future.