Yep, Aaron Shepard's Sales Rank Express is considered to be the widget for tracking Amazon sales ranks. I don't really use any of them, however, simply because they change constantly (by the hour) and don't mean much unless you are trying to do a specific campaign. I look at it as similar to the whole pagerank hoopla: trying to get a higher pagerank is not a smart option. Rather, you time is better spent writing great content and building links, pagerank will follow. However, it is a good gauge to track progress.