Many websites employee a centered table of a width of just less then 800 , and this introduces blank margins to the left and right. This allows them to and optimize that centered table area with all the sites content for a screen resolution of 800 x 600.
It tends to introduce a lot of wasted space for larger desktops, and the average user will have to employe more scrolling because your content tends to be taller than wider, but its a better way to go than to optimize for 1024x768 or larger.
TJR