For PowerBook G4 5,6 (1,67 GHz and DDR1 model) the Mplayer results are:
640x276 mov: 14,282 s 1280x544 mp4: 35,643 s
for a quick comparison - PowerBook 5,8 (1,67 GHz and DDR2 model) results are:
640x276 mov: 15,021 s 1280x544 mp4: 36,631 s
So it looks the results are pretty much the same (actually slightly slower for the DDR2 model, but I believe if the test were repeated a few times more we could get the same result).