These are all the codecs you need:
Divx - Necessary to properly read the audio from most AVI files. No need to dance and prance with "doctoring" them before opening. (Edit: I just noticed that the Divx web site says it does not support QT7 or Tiger, but it works fine for me.)
AC3 codec - for decoding audio in the rare AVI that uses AC3 (the codec used on DVDs) instead of mp3.
WMV Player ($9.99) - for handling WMV files in QuickTime.
There's also
3ivx, which I used to recommend in addition to Divx. However, it's pretty much worthless now. 3ivx is faster at decoding video than Divx, but ONLY if you're using QuickTime 6; 3ivx is for some reason much slower with QT7, to the point where playback is choppy even on GHz+ G4s. This issue has been acknowledged by the 3ivx guys, but has yet to be fixed. Also, it used to be that if you installed both, you'd get Divx's audio handling + 3ivx's then-superior video handling, but as of Divx 5.2.1, this is no longer the case (Divx will handle everything anyway). So the only reason to install 3ivx now is if you're dealing with 3ivx video, which is very, very rare.
With these codecs installed, I very rarely need to use VLC. VLC is a great program, but it's no substitute for QuickTime.
If there's no "QuickTime" folder in your Library, just create it, and drop the codecs in there.