kenn michael said:depending on the speed of your processor, FCP will slightly lower the playback res (on your computer monitor) to be able to play full framerate. that's why it looks worse on your G4 than on your G5.
Quicktime Player gives you a choice to play full res, or slightly lower res (very slight difference) by choosing an option in the playback settings of the file.
Very well, but that still doesn't explain why a g5 edited quicktime file struggles when playing back on the powerbook, while the powerbook edited file plays smoothly...