If you have a Windows PC, download the latest
MobiPocket Reader. Amazon owns MobiPocket, and the Kindle is using a variant of the MobiPocket Reader for handheld devices (similar to the one in the "others" section of the download page). The Windows Reader has more customization features than the Kindle, but it should give you a good idea of what source files will look like on the Kindle (particularly if you set your window size to about 600 by 800 pixels).
Whispernet conversion is probably similar to the import option for the MobiPocket Windows Reader, the output is a PRC file which will work on the Kindle. In fact, this is a free alternative to using whispernet conversion (copy the PRC to your Kindle via USB).