Can you refresh our memories on that one?
I use an HDMI cable to feed from the DVI-D port of the MythTV box (with a DVI-D to HDMI adapter dongle) into an HDMI port of our HDTV (Viewsonic).
I did this with my old Shuttle box with an old ATI Radeon card -- I don't remember the model number. It used to be an 8900, but I had to replace it with something else.
I seem to recall that I couldn't get a decent resolution, or decent quality out of it. The VGA connection was better. Reading the manual for the TV implied that it didn't accept DVI-D inputs, even with the dongle, over the HDMI input.