Hi Saxxo. Welcome to the forums.
Why is the content on the page (except the menu) jumping down in IE 5 and 6?
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.)
I can't figure out why I am not able to make the columns wider
Your container has a fixed width:
margin: 0px 0px 50px 0px;
<font color='"#FF0000"'>width: 678px;</font>
That sets the limit you can work within.
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.