I've been asked quite frequently what the current "standard" is in width when designing layouts for web design. While there is really no set "standard", 960 pixels is one of the most frequently used dimensions, and it's the one I recommend. There's even a website dedicated to the 960 pixel grid layout called the 960 Grid System. Some may even argue that it's now gone beyond that.
So why 960 pixels? The typical user now has a screen resolution of at least 1024 pixels in width. Looking at the stats for the UCLA Gateway (http://www.ucla.edu) from July 2008 to July 2009, only 2.7% of our users are still using a screen with a resolution of 800 pixels in width or less. 960 pixels accounts for the extra padding a browser scrollbar and its other elements can incur, depending on the browser you use. The actual view space will be this extra padding + 960 pixels. Also, 960 is easily divisible by 3, 4, 5, 6, 8, 10, etc. So when designing individual elements within your layout (navigation, sidebar, images, etc.), these will give you nice dimensions to work with.