19 inch flat screen here...native is 1280x1024...which is where I'm at most of the time.
When building, I resize every page to make sure it looks decent at the most common sizes. Since I work in FF almost exclusively and have the developer tools installed...I can switch to 800x600...or 1024x768 at the click of a button
That said, pages that are specifically for webmasters, 1024x768 becomes the primary choice with 800x600 secondary.