I recently had to convert some RMVB-videos to AVI. I searched for a way to accomplish this and found a blog entry about MEencoder. I tried the exact procedure described using Ubuntu 10.10 and it worked like a charm.
First of all, install MEncoder
sudo apt-get install mencoder
After that, as follow below…
mencoder -oac mp3lame -lameopts cbr:br=64:vol=2 -srate 22050 -ovc xvid -sws 1 -xvidencopts bitrate=500:max_key_interval=120:vhq=4 -ofps 30 -vf scale=640:360 /you_path/file.rmvb -o /your_path/file.avi