I fixed it!
First, thanks to you who offered assistance.
For those of you who read this in the future and run into problems with "Sidebar Down in IE," I have a couple of tips.
The most common problem is something too wide in your sidebar. For instance, if your sidebar is 200px and you try and ad something that's 250px...sidebar down.
My problem came about when I tried to change the dimensions of my 3-column theme.
The original theme had a right and left sidebar that were each 200px wide and a content area that was 510px wide. That's 910px of usable space total with 40px (950px header)left over for margins and padding.
The left sidebar and the content area are in one container (originally 730px) and the right sidebar is in another container (originally 220px).
I needed to make the left sidebar 300px. To do this I had to increase #left to 320px and decrease #right to 630px.
The math should have been right (and it is) but still my left sidebar was being pushed below the content.
So, here's what you do if you run into a sidebar down issue: Using Firefox, download the Firefox web developers toolbar. Once it's installed, click the button that says "Information." You'll see a lot of options. Click on "Display Block Size" or "Display Topographic Information."
This will give a nice outline of all the blocks on your theme. Is one spilling over into another?
In my case it was the .navigation. I hadn't changed that number to match the new size of my left sidebar/content area container. I changed that and everything works great in practically all browsers including IE6 and 5.5.
For those more experienced than I, I'm sure you could care less. But, for those on my level - and by the number of sidebar down posts, there's obviously a lot - I hope this helps.