I see a whole lot of designers have problems with layouts especially when it enters the 3 columns or more columns design.
Also the 2 columns still seems to be difficult for some among us.
First of all a few years ago I only was designing with a WYSIWYG editor.
In iE everything was looking wonderfull.
Untill I met Linux.
I checked my site with a browser in Linux and everything was mixed up.
From that moment on I decided to check my pages in different browsers.
And the result became better (not 100%) but it made me more confident.
because pages became viewable for more people, and that is what counts.
Now for all designers among us Firefox has a great "Inspector" build in called "Dom inspector".
What you must do now to find difficulties in replacements of your content, sidebars or other ads is placing borders around the elements who are causing the problems.
You can remove the borders later......
And more better place borders around all possible elements who needs a special placement.
The Dom Inspector from FF will surprise you but also will be a great help for all of you ( I hope! ofcourse).
Then one more thing test your site on Opera IE Firefox and Netscape minimal and if possible also with the KDE browser from LINUX.
Your results will become better.
Also check out pages about CSS and don't forget the W3C site.
There are also some great sites on the net giving very specific tips to work around problems.
Especially IE is the most horibble browser you will find out soon.
I hope this tip will help some desperate developers among us.