Some users have large monitors which support screen resolutions in excess of 800 pixels wide by 600 pixels high. Some users are limited to smaller resolutions by their hardware. For some users, reslutions above 640x480 make text too small to read. Design flexible pages that look work correctly at multiple screen resolutions. Never put primary content or navigation on the right hand of a screen if it will not be fully viewable at 800X600.
Users are unlikely to scroll horizontally to view content.