Crappy movie can be played smoothly on half the speed machine you mention. But such movie is 640 pixels wide Xvid/DivX thing.
It is a great information to share with us about what kind of movies are you planing to use.
The HDTV stuff (typicaly TS stream with mpeg2 ac3 movie inside, bitrates from 15 (low IQ crap) to 45MBi (you can got bonner from this IQ :) ) and 1920x1080i at 29.976 fps require something over PR3200+ with decent gfx card (9600 or 6600) that support the decoding greatly, othervise almost insane sounding 4500+ machine is need to get rid of jerking.
A middle ground could be a WM9 video, 1440 pixels wide and requiring above
PR 2500+ to play w/o jerking. Very fast (talk aout X1300+ in Radeons or 6800LE in GF cards) GFX card to make the decoding run at about 50-60% of CPU time on my PR3800+ machine is a requirment as well.
Low end could be a DVD image, with is 720 pixels wide, widescreen enlarged to 853 pixels wide mpeg2 stream at top 10MBi bitrate and bellow average image quality. Expect 40-50% CPU time on PIII/800Mhz to be need - assuming that the card can open a YUV 422 overlay (often incorrectly mentioned as DVD decoding support...)
