Couple things here. First, as you noted, you can definitely rearrange everything. Move boxes from one side or to the other, turn off one or both columns. This is all done through the control panel.
To take that a step further for sites that really need things narrow. Pretty much all the boxes you see are available for use anywhere on your site just with a simple include. Say you really wanted to have the Calendar, Who's Online and Forum Stats boxes, but you really have no room for sidebars. You could actually do a quick change to one of the templates and include those on every page or maybe just on a few pages.
This isn't something that's built into the control panel, but the way things are designed makes this very flexible with some fairly easy changes. We'll definitely do up some documentation for what needs to be changed to accomplish things like this.
