Not so. In my visitor population it is currently 3.6% which represents a considerable number of people. James
Yahoo's Page width is almost 960pixel. and I think that is the best size for big site. It depends on Site's Content. Like Forums should have 100% screen size like digital point.
Think about your target audience. If you're aiming at a more tech savvy group, like people in this forum, then the number of people running at 800x600 is likely to be insignificant.
you may use up to 1200, now days most people have a good enough comptuer and monitor for a highter res, the 800 days are long gone;