That'd be my guess as it may be that the code for handling those encodings are different between the cards and so it shouldn't try to run the 6800 card like a 5200.
I recently swapped out the video card on my Sony Vaio PCV-RZ54G Desktop computer in getting ready for Vista. The original card I removed was an Nvidia GeForce FX 5200. I upgraded to a PNY Verto Nvidia 6800 GT. Everything went smoothly and the new card seemed to be passing all tests until I tried to view some video clips. AVI's, WMV's, and MPG's run but you cannot see the video although you can hear the sound. DVR-MS files run perfectly displaying the video and you can hear the sound. Any ideas on how I can get the 6800 GT card to display the AVI's, WMV's, and MPG's will be greatly appreciated. I'm totally perplexed since they all run properly if I switch back to my old FX 5200 card.