Sorry Rob, I got distracted by work ;-)
There are still even narrower screens? God!…. How nerrow than it might be?
Certainly there are.
Resolution: 320 X 480 HVGA
OK, so that's an Android cell-phone, and I'm not seriously suggesting going quite that far as fixed width, or even that your site should look good at that size. But it is worth noting that smart phones generally do not identify themselves as mobile devices, and the narrower your site can adjust to, the less trouble you will cause for the increasing number of phone-based browsers.
Then again, my screen (one of them) is 1920 pixels wide; any web page becomes hard to read at that width, so I never have my browser full-screen and its size depends on whatever else I might be doing. Usually 800-1024 pixels would be readable, but it would not usually be optimal. It would not be unusual for 1024 to be too wide for my window, whereas 800 would usually waste a significant amount of space.
I built this page as an exploration of how extensible a page could be and still be easy to read. It's a range of about 450-1800 pixels wide and the extreme ends of that are arguable. So 800-1000 is the sweet-spot, and that is where most web-pages are designed.
To my mind, it does not really matter how your page looks at 400 pixels or 1600 pixels, so long as it degrades gracefully. Just design it for 800-1000 pixels wide; that's how 99% of your readers will see it, and the rest will accept a little less perfection in exchange for using their screen space efficiently.