But having spent an hour or so figuring this out (and having just tested it with good results), I think I'm on a good track, so I figured I'd share this for anyone new to this topic. If anyone on this forum who has some experience with source ordering can chime in and add anything I may have missed, please do so and I'll include that in this post for anyone looking for this info.
Let's say you have a default layout that has a content area on the left and a sidebar on the right. And let's also say, for the argument sake, that you changed your content area to take up 9 columns, and your sidebar to 3 (for a total of 12):
That means that you have to push the 9-collumn-wide content area by 3 columns, and pull the 3-column-wide sidebar by 9 columns.
In short, content = push3 and sidebar=pull9 (notice you're specifying the amount of pushing and pulling, as opposed to specifying the width of an element)
In Montezuma Options > Main Templates locate the template you wish to modify (or create a copy and assign it to a testing page before you commit) and hunt down the opening <div> tags for each of these two elements. They will look something like this:
<div id="content" class="col9"> and <div id="widgetarea-one" class="col3">
<div id="content" class="col9 push3"> <div id="widgetarea-one" class="col3 pull9">
But most importantly, as far as search engines are concerned, the content will continue to load before the sidebar.