Split your html at the line that reads <div id="content">. Take everything above that place it in a file name 'main_1.html' or something similar. Take everything that's left and put it in a file called 'main_2.html'. main_1 is your header menu and right sidebar. These things will not change/ main_2 is your content that will change.
Write a php file index.php containing this:
If you don't want what's left after the </div> for <div id="content"> Make a third file and change the index.
As for two ~ three columns. Right now your content is 552px wide. You could plave to columns at 250px wide in your content div and be ok. If you want to go to three columns I would suggest redoing the page at a width of 1000px.