Yup, they truly will play MOST (there are exceptions of course) of the videos.
Also, if you search, say google, you'll find that there is also a thing called DivX Doctor wich alters the file so that it can be playable in Quicktime.
While the latest method allows you to play the file in a player that, in my opinion, is more stable, the resulted file will be twice as large and you'll end up using A LOT of disk space.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

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