Hi Saxxo. Welcome to the forums.
There will be a technical answer to that question, but the layman's answer is—Who cares? Nobody uses IE5 any more, and IE6 has something like a 0.4% userbase, mainly because of China. (I can't answer your question technically, but truly, I wouldn't worry about those old, dead browsers. We've all moved on.)
Why is the content on the page (except the menu) jumping down in IE 5 and 6?
Your container has a fixed width:
I can't figure out why I am not able to make the columns wider
That sets the limit you can work within.
margin: 0px 0px 50px 0px;
Lastly, I tried adding this to #columns but the bottom dotted line won't show ...
That will make the columns div wrap around the floated divs inside it (which otherwise hang out of it). Also remove the height setting on the #columns div, as you should never set heights like this.