Are you using OSX or OS8/9? Might I recommend you get VLC which has built in codecs to play almost all video types.
http://www.videolan.org/vlc/ (choose nearest mirror site) To see what it can play see
http://www.videolan.org/vlc/features.html
I use it for some video playbacks & QuickTime for others by setting the defaults for those types of video files. VLC is much better & reliable for previewing however.
What plug-ins should you get for QT? Not sure about what apple suggests. I'd suggest
DivX, -
XviD, -
3ivx, oh & divx doctor. I do know one of the ones apple suggests is very good for encoding.