400MHz might be a little slow for video playback, but I'll let someone who owns a machine at that speed comment on their own.
All PowerMacs are very, very cheap, though, so I think you'll get the most bang for your buck by picking up one of the later models (1.25GHz or 1.42GHz).
I agree with Matt, don't buy that 400MHz G4 PowerMac, until you shop around, as you can probably find a faster G4 model for the same or less money. I got a Dual 1.25GHz G4 model (the MDD model) for less than $40 and was able to pick it up locally, so I didn't have to pay any shipping either.
The 400MHz model will run MorphOS2.7 okay, but don't expect to playback any video content smoothly.
Shop around.
