ExtremeTech just posted results of their tests comparing CPU utilization across recent ATI (Radeon X1800 XT) and nVidia (GeForce 7800 GTX) cards for three codecs: DivX, H.264, and WMV9.  The results?

For 720p content WMV 9 plays back at typically 25-40% CPU utilization for the clips tested.  H.264 on the other hand takes 80% (using ATI's acceleration) and on other systems even spike to 100%.  Note however these are different clips than those used for WMV 9 testing. Playing back 720p DivX clips results in 50% or so CPU utilization

To quote:

"H.264 acceleration has a long way to go before it's ready for prime time. Even with ATI's hardware acceleration, it's way too CPU intensive. What's more, ATI needs to work to offer acceleration on basically all popular H.264 decoders, the same way their DVD acceleration works with DVD decoders."

Also interesting to note DivX playback performance of DivX player vs. WMP:

"The performance of the GeForce 7800 GTX when using the DivX Player is atrocious, at 75-80% CPU utilization. Under Windows Media Player 10, it's right around 50%."

One thing's for sure, there's still a lot of work to be done in this area. 

"The video landscape on the PC is still far too big a mess. There are too many codecs, and sometimes too many software providers making decoders (we found a dozen H.264 decoders and at least as many DVD decoders in 10 minutes of Google searching). Some are accelerated, some are not."

I have some ideas on how to fix this (that don't involve "destroying" the competition thank you) but I'm interested in your thoughts first?