Archaea, correct me if I'm wrong, but you indicated before that you were playing "1080p sources" (WMV files). Have you actually played HD DVD or Blu-ray movies directly (from disc) or ripped (but kept in their native format)?
I've played several 720p DIVX and WMV files (I can't recall if I've tried playing any 1080p files) with just the stock 6150LE on-board graphics and they play great. But my understanding is that HD DVD and Blu-ray are using much higher bitrates. The 8500/8600 cards have specific technology that allows for playing these formats. Theoretically, I might be able to play HD DVD native files with even my on-board 6150LE graphics if my CPU was fast enough, but that doesn't mean that much/any of that is being handled by the graphics card.
I've played several 720p DIVX and WMV files (I can't recall if I've tried playing any 1080p files) with just the stock 6150LE on-board graphics and they play great. But my understanding is that HD DVD and Blu-ray are using much higher bitrates. The 8500/8600 cards have specific technology that allows for playing these formats. Theoretically, I might be able to play HD DVD native files with even my on-board 6150LE graphics if my CPU was fast enough, but that doesn't mean that much/any of that is being handled by the graphics card.