Here's an interesting article from AnandTech talking about the new GeForce 8600's video acceleration features. It features H.264 acceleration. The article analyzes the performance win from doing acceleration in hardware. It also gives some statistics on VC-1 (standardized WMV) acceleration. It compares the CPU usage with NVidia, ATI, and no acceleration. The results are, shall we say, stunning. While this shouldn't be your only input for buying a video card, if you want to play video, these data points are certainly important.
While I'm on the subject, here is an interesting post from one of my team's program managers on choosing display cards for video in Vista.