I don't know much about video transcoding but I spent a few hours hacking around with some of the programs and examples in the FAQ trying to get a flv converted for my sansa c240 with out much luck - so when I finally got one right I wanted to share that I had success using the ffmpeg GUI "AVANTI" which gave me the following CMD:
ffmpeg.exe -i "c:\invid.flv" -vcodec mpeg2video -aspect 1:1 -s 132x80 -vb 200000 -f vob -acodec libmp3lame -ab 192000 -ar 44100 -ac 2 -y "c:\outvid.mpg"
Hope this helps someone