It really depends. Most times though, I’d use a fixed 960 grid layout. Otherwise I’d do a liquid layout with max-width and min-width set (with IE6- hacks in place).
It isn’t the width of the entire web page that is most important. It is the width of the individual columns. Once columns go over 35em in width they become difficult to read and people lose track of which is the next line and will either reread the same line again or skip a line. If you want to fill the viewport at wider sizes then you need more columns (or make the text bigger so that 35em takes up more of the width).
The other thing to make the site more user friendly is to make sure that the content stays within the width of the available viewport. For example this forum would be easier to use if it didn’t have a horizontal scrollbar.