I tried both MP3Splitter and MP3 DirectCut. I found that MP3 DirectCut seemed to give me cleaner files -- MP3Splitter left some minor "skipping" artifacts for some reason.
Three minute files seem to work well for my Sansa Express, and the transitions from file to file are mostly unnoticeable. I also figured out how to get the Sansa's playlist function to work for me (not the most intuitive process, I'm afraid), so it will actually play the book files in the order I specify rather than shuffled (that was a pain).
The bottom line is, it looks like you two have put me onto a method that will, indeed, work for me for non-Audible.com audio books. Thanks!