You can safely assume 600-700 pixels width. No more. I Sure don't run my browser in 1600x1200 just because my desktop happens to run in that resolution.
Hmmm. You're imposing maximum resolutions without necessarily considering the effects, and the overall design. I believe that the design of the page is such that if you browse with a more narrow browser than the design calls for, the only information 'lost' is a screenshot. This seems like a fairly decent design tradeoff to me. (Of course, if the design doesn't work this way, maybe it should, Jer <g>).
Jer