The cpu load from x.org for me was above 50% when watching high def channels. Since mythfrontend was using the other 50%, I was getting audio stuttering and other problems...
This worked for me on an nVidia 6200:
First, upgrade to the latest nVidia driver using instructions from here:
http://mysettopbox.tv/phpBB2/viewtopic.php?t=13443&highlight=
(Note, read the whole thread first)
Then, edit the /etc/X11/xorg.conf file and add the line
Option "UseEvents" "True"
To the "Device" section.
I got the information from here: http://www.gossamer-threads.com/lists/mythtv/users/242186?search_string=useevents;#242186
Basically, the problem is that the older nVidia driver is doing a busy wait when waiting for v-sync. The new driver uses a different method that's not processor intensive.
Now, when watching HD content, my x.org hovers at around 7% cpu load.