About a year ago i remember one of the major hardware review sites (mightve even been anandtech?) had a big review on the htpc technologies like purevideo vs the upconverting dvd players. The result was that the upconvert technologies from ati (Amd) and nvidia were quite a bit better than even the best upconvert players.
Now, this might have changed with the hd-dvd players and blu ray players. One thing i WILL say, however, is that going from my htpc (e4300, 2gb ram, xbox360 hddvd drive) to a toshiba a35, is that the menus were MUCH smoother on the htpc, and felt alot more responsive.
Granted, getting the hd-dvd's to play back in the first place was the hard part, but after that, everything was gravy.