Consider your audience. If this website caters to an audience that is only likely to run their browsers* at 1280x960 then it is safe to design for that resolution. If this website is intended for all audiences then the maximum width should not exceed 800 pixels.
*Browser not monitor. I run my monitor at 1280x1024, but my browser size is set to 1024x768 so that I can keep things like chat windows, rss feeds, clocks, etc. visible while surfing.
If you want to get an idea of what a website will look like on lower resolutions without changing your monitor resolution then download the web developer toolbar for Firefox. One of the options allows you to specify the browser window size. Keep in mind that what you see in that window is what occupies the entire screen of monitors running that resolution.
https://addons.mozilla.org/en-US/firefox/addon/60
Hope that was helpful.