Currently, web designers are advised to support browser resolutions of widths 1024px and greater based on current browser usage data. The idea is that you want your site’s design to look good, and not require a horizontal scroll bar for those users. But screen resolution and browser width are not the same thing, because of browser chrome, scroll bars, and because many users browse without their window being maximized.
Common advice is that for the 1024 resolutions you have a maximum width of around 960 or 980px to work with. But if you really need to get every last pixel in, I can confirm test results from Windows XP running at 1024×768, with Internet Explorer 6.0 fully maximized, gives you a width of 1007px to work with.
* Of course, fluid layouts are also nice.