I feel safe using 1000px.
If you are worried about different devices look up responsive design and media queries. This is a sample tutorial involving DW:
Google labs - Browser Size
If you want SAFE, use Google Analytics or some other web stats reporting to determine who the majority of your users are. Then build accordingly.
Responsive Web Design seems to be less about theory and more about necessity now.
Thierry Koblentz has an interesting take on it: