Do you even know how HD videos are decoded? CoreAVC and FFDshow (and VLC if you want to count that in) all do not support GPU decoding. The only one that does, is the Cyberlink PowerDVD decoder via DxVGA. But let me tell you, the 7900GT in the system did nothing much even using the Cyberlink codec. CoreAVC and FFDshow performed far better.
As for Nvidia PureVideo, that's a PureJoke. Mpeg2 decoding is again far better using FFDshow, with much better control over the output.
So during HD playback, GPU was NOT used AT ALL. The guys at CoreCodec promised GPU decoding in upcoming versions, but that never happened. Now they claim that the CPUs are fast enough so GPU decoding is not required. Note, that it takes just a Intel Core Duo 1.8Ghz+ to decode 720p and a Core 2 Duo 2Ghz+ to decode 1080p comfortably.
Maybe in future Windows7 will bring about GPGPU (gx keeps mentioning that) which will enhance the experience out of the box, but for now, GPU is only used for Aero effects and Gaming.