The black screen issue was becoming a big problem and vmc started crashing when I tried accessing recorded tv, so I reinstalled vista last night and didn't install the tv pack.
So far, I haven't seen the black screen issue. Also, I figured out why hardware acceleration wasn't working!
My vista performance index was 1.0 because it didn't have a driver for the 8600gt. Once I installed the nvidia driver and reran the test, aero turned on along with hardware acceleration! CPU utilization for ota hd is a little higher than it was with mce2005 (~10% vs. ~25%), but I can live with that. I may try the powerdvd8 decoders to see if they are a little better. Also, the picture is much better with ha enabled. There was a lot of noise in the sd recordings that weren't present in mce2005 or with vmc and ha enabled.
Additionally the fans have returned to their normal, slow and quiet speed.
I miss the easy qam setup with the tv pack, but it surely isn't worth it for me due to stability issues.
Thanks for your help!