.. a super-fast processor coupled with a super-fast graphics card with the ability to assist the CPU with the video decoding. I suppose you might also be able to find a dedicated decode accelerator board. It's pretty expensive (in CPU power) to decode high resolution H.264 ..
Any NVidia chipset from the past couple of years
can decode H.264 in hardware, with minimal CPU usage. Heck, even my lowly fanless GF6200 card here can do it.
I'm more interested in MPEG2 decoding, which "duke" currently does in software here. Over the holidays I may upgrade it to latest MythTV-svn (bleeding edge), which has good support for the NVidia hardware decoding stuff.
Cheers