Two points to consider:
1. projection settings and regular computer settings will probably be different. I think most people set the projectors resolution to what it handles the best, and probably have the primary computer monitor higher than that (make room for all them side bars and such).
2. What the resolution of the display are and the dimensions of the browser window can also be two different things. As many system get wider displays, more people browse with their browser not maximized. I am constantly amazed at how many people I know even at lower resolutions always open a browser window not maximized, and then have to scroll all over.
That being said, Probably best to design for width of 1024.