itunes movie file formats

I have a bunch of movies on my harddrive in avi format that Quicktime will play, but itunes will not recognize. Can anyone recommend a good program for converting them to mp4? I have MPEG Streamclip, but this doesn't seem to work--after a legthy process of conversion, only audio will play, not video. I think all the conversion settings are correct for itunes and ipod specs. Any ideas?