All but the newest codecs are available on OS 9. Here's what you definitely can't play:

- H.264

- VC-1

- WMV 9 or later

Thanks to QuickTime 6, you do have access to AAC audio, as well as regular MPEG-4 video.

The DivX people have made it very hard to download DivX for OS 9 but I managed to find it for my 1.6 GHz G4 rig. I'll upload it if anyone needs it.

1080p is simply not possible on a Dual 450 G4 though. Probably the best you are going to do is widescreen 480p - and that's if your decoder is using both cores.