If you have a later model 8 or 9 series nVidia graphics card, you can take advantage of its CUDA features to offload the h.264 processing of .mkv files from the CPU to the graphics card. You will need CoreAVC 1.9.5 to make use of CUDA.
Here was my setup before:
CPU = 1.8 to 2.0 GHz Intel Dual Core
Video resolution = 1080p
using CCCP = 80-100% CPU utilization (either it stutters and loses frames, or plays really choppy)
using CoreAVC = 50-90% CPU utilization (very little to no stutter)
using nVidia 9500 video card with CUDA + CoreAVC 1.9.5 = 15-35% CPU utilization!!
(Flawless!)
If you want a good 1080p test video, try Chicago in 1080p. Check mo yun 1st song na All That Jazz at yun last song of the movie. It will really tax your system! This is my benchmark for setting up a 1080p-capable HTPC.