Thanks for the suggestion, eping. That's a very helpful article.
I didn't know about Mobipocket when I began the conversion from MS Word to MOBI. If I had, I would've tried that! Instead, I used the command-line program KindleGen.
I probably went about it the hard
way, but I saved the file as filtered HTML from MS Word and then I cleaned out all the garbage code manually. Fortunately, I already knew HTML! Then I manually created .opf and .ncx files using examples that came with KindleGen and created the MOBI file using that program.
I just ordered a Kindle 3G, which will arrive on Monday, so I'll be able to test the file then.
I'm still tweaking things, but it looks very good on Kindle Previewer.
KindleGen and Kindle Previewer