I've seen a trend lately that I really like and have used w. client sites...
You fit the content of the site so that anyone viewing with 800x600 res will see all of it, but provide and extra column to the right of it for ads or other supplemental items.
WashingtonPost.com does this very well. people w. 800x600 res will have to scroll to the right to view but for everyone else the layout fits perfectly on the screen. it allows for much more content,etc on a page
I personally hate coding or designing for 800x600 but still do it.
One of my bigger regrets with my sites is that I made them fixed width and the width is smaller than what suits my needs these days. Originally it seemed perfect but as we added more and more content and features I've had a hard time making room for everything... kind of painted myself into a corner, so to speak