Do you mean that you want to center it vertically or horizontally? You said you specified the height to 800 pixels, and wanted the div not to stretch when the width was greater than that...
To center horizontally, you can use the margin: auto; attribute in CSS. Also, you'll have to make sure that the body and html elements don't have any margin or padding:
html, body { margin: 0; padding: 0; }
#centeredDiv { margin-right: auto; margin-left: auto; width: 800px; }
