Ok guys... There is some confusion about the codecs here....
First of all understand MPEG-4 (Motion Pictures Expert Group) is a guideline for Compressing video and audio for Streaming other purposes.
Divx and Xvid Follows MPEG-4 Guidelines for compressing Video (But not entirley, So some divx movies are not fully compatible with MP4 video...).
And AAC = Advanced Audio Compression. Its MPEG-4 Standard for Audio... it has nothing to do with Divx or Xvid.
Codecs that Im aware of that uses MPEG-4 Standards
Divx
Xvid
Nero
3vix
Real Video
WMV 9,10 (Im not exactly sure on this one)
So any player that can support MPEG-4 video can easily support Divx and Xvid (as the extra stuff they have are negligable..)