The W3 Schools has an in depth report of user browser stats. This is an important read for all web designers.
The statistics show that Internet Explorer 6 is the most common browser, XP is the most dominating operating system, and most users are using a display with 1024×768 pixels or more, with a color depth of at least 65K colors.
However, 14% of users still use 800×600 monitor resolution where the maximum width of a web page would be 760 pixels. Also, many users do not surf with desktop windows maximized, so your design could still be unintentionall cutoff horizontally requiring the user to maximize their browser.
What should you do? Research your specific audience then design the width of your website accordingly.