Decoding H.264 is much more intensive than MPEG-2. I've heard that CoreAVC is the fastest H.264 decoder, might be worth giving it a try to see how it compares with ffdshow.

I can play 720p x264 files with no problem on my PC (Athlon X2 4200+) with Media Player Classic Home Cinema Edition, although to be honest I'm not really sure whether it's using ffdshow or the Cyberlink decoder.

I've just got a DVI to HDMI cable so will be testing out some HD files in the near future - although time is a bit sparse at the moment!