770 can play .3gp files which contains MPEG-4 standard with .amr or .aac audio but there is limitations what kind of files it can play, e.g. resolution, Framerates...

I think .avi can contain many kind of codecs and formats just like .mp4

nice links about MPEG-4 and .mp4
http://forum.doom9.org/showthread.php?s=&threadid=62723

http://forum.doom9.org/showthread.php?t=73022

http://en.wikipedia.org/wiki/MPEG-4_Part_14