Okay, so say this:
Div 1 is variable width
Div 2 is fixed width at 400px wide.
You place Div 2 inside Div 1 (<div id="1"><div id="2"></div></div>)
When displayed, this will be fine as Div 1 can stretch as needed to fit its contents.
HOWEVER
You place Div 1 inside Div 2 (<div id="2"><div id="1"></div></div>)
When you have content in Div 1 that would make it expand to say 800px wide, what you'll get is one long column 450px wide. So sometimes variable width items inside fixed width is fine (like having a side-bar with mini-news items. However, sometimes it can backfire.
Basically, I was just saying that you need to think about your layout before you go jumping into it.