The challenge is made more difficult if you are not using Windows. There are a few tools out there though, but all require Mplayer, and on a mac you will need Mplayer for OS X - which is rarely updated. You will need to get the old version which comes with mencoder and copy it out of the application package.
This is the command I used, which worked fine for the iRiver Χ20:
./mencoder pls_sunflower.avi -quiet -vf scale=320:240,expand=320:240 -oac mp3lame -lameopts abr=128 -ovc xvid -xvidencopts bitrate=450 -of avi -af resample=44100 -srate 44100 -ofps 10 -o test3.avi
Unfortunately that didn't work too well on the E10. But here are some links to help with that:
- video encoding with mencoder for iriver E10 settings
- a blog post on encoding video with mencoder for the iriver E100
- iRiverter, a java program for converting video for the iRiver, but doesn't seem to work on the Mac.