To be fair to Damien’s original comment, and contrary to what I stated, the video card DOES have an effect on very high resolution video playback:
http://www.xbitlabs.com/articles/video/print/video-playback.html
As long as you’re not using something truly horrible like integrated video, the difference in playback between video cards is fairly minimal, though. Compare the very high end NVIDIA card with the low end from that article:
CPU load (min/avg/max), on an Athlon FX-60 (dual 2.6 GHz)
DIVX 1080p
GeForce 7900 GTX: 26, 37.5, 43
GeForce 7300 GS: 30.5, 38.9, 50
H.264 1080p
GeForce 7900 GTX: 7.2, 17.5, 35.9
GeForce 7300 GS: 14.1, 22.4, 36.7
VC1 720p
GeForce 7900 GTX: 3.1, 17.3, 26.6
GeForce 7300 GS: 2.3, 17.6, 27.4
WMV HD 1080p
GeForce 7900 GTX: 5.7, 12.7, 20.3
GeForce 7300 GS: 7.0, 12.7, 21.1
The only variable in these tests is the video card, so any differences should be directly attributable to the video card.