Prax – You use CSS for this. Basically, float the main DIV left or right with a width. Give the sidebar a new block-formatting context by, say, using overflow. IE 7 and older need “layout” – the sidebar then takes up remaining space at the side of the main DIV.

Example:
#main {
float: left;
width: 62%;
}
/* only needed to give old IE "layout" */
#sidebar {
display: inline-block;
}
/* cancel inline-block but not "layout" */
#sidebar {
display: block;
overflow: hidden;
}

HTH.
Cordially, David