Like the others, I aim for a 700, but attempt to design it so that the content area can expand if the screen is larger. If someone has some nice screen real estate its always good to fill it up.

If you use Google Analytics on your site, one of the stats it records is the screen resolution. Most say that theres less than 10% of internet users using 800x600. Some of my clients systems use 800x600 so the stat is skewed to 15% on my current project.

It also tracks geography of customers and their net connection, (DSL or Dialup etc.).

While AWSTATS doesn't require any coding on your page, Google Analytics runs a brief script at the end of your HTML which logs the usage and details to Google. It can cause a slight slowdown now and then, but my clients have enjoyed the very pretty and detailed output.