Despite the overclocking I did on my DVR computer, I was disappointed that it's choking a bit on the HD playback, though I'm not sure why as it was acceptable yesterday. When I put it up full screen I get some tearing and interlacing artifacts. If I watch it at its native 720p size in a window it's fine, but the computer is having a hard time upscaling it to the full 1920x1080.
I've spent a fair amount of money on my DVR PC, though way less than a grand. After 16 months, overall I'm pretty happy with it. I'm hesitant, however, to do any upgrading. I have this feeling that Apple has something cool in the pipe, as Front Row is a great application that does iTunes, DVD and photos. So I'm kind of in a wait-and-see mode regarding the "living room box" that pundits have been predicting for years. It's getting there, but it's not totally there yet.
While I know you don't have the open source (MythTV) version, I did find this when I was doing some research on a PVR.
http://www.mythtv.org/modules.php?name=MythInstall
If you look at the hardware section, they give a brief rundown of the CPU needs for the different resolutions. BeyondTv _might_ be similar.