Originally Posted by jvincent
Not 100% true.
If you are using S/W post-processing, aka ffdshow, the CPU is important. If you aren't then the GPU, and drivers running on them, make a big difference.
Only if you choose a GPU under a certain baseline - and all HD-compliant display cards are generally beefy enough to handle most of those tasks (a lowly FX5200 can do DXVA and VMR9, whereas ATI's side requires a bit more with a 9500 being the bare minimum card).
I'm assuming the OP wants to buy a new card that's widely available now, so his retail options are pretty much 6600GT/9800P in the $150-200 range. Should he opt for a lower card like the 9250, then for better quality and dependable performance, I would use CPU post-processing.
I'm currently running full DXVA acceleration, using VMR9 with TheaterTek, i.e. Nvidia decoders, on my AIW9700Pro and the upscaling is as good as it's been in a while. I've turned off ffdshow and haven't felt the need to go back.
My experience with Theatertek is limited but I'm not sure that you mentioned what does the actual upscaling. On my setup (XP3200+, 9700P, 46" DLP on DVI>HDMI) I can definitely notice the differences in visual quality between standard GPU-enabled processing and ffdshow processing.
Lord of the Rings (Fellowship Extended) is one movie that I use to test my settings; the scene where the Fellowship first encounters Galadriel can clearly show inadequate upscaling. The dark forest background will show transfer noise that hasn't been cleaned up while skin tones on Galadriel's cheeks can show too much processing if they lose their gradual transition. Using VMR9 and in full-screen mode (not true upscaling), the noise remains very noticeable in many players including Theatertek, Cineplayer, and PowerDVD. Also, "upconversion" using anything besides ffdshow results in broken up skin tones.
imo this applies to some other material like the X-Files or Stargate SG-1 tv series which use lots of high-contrast scenes, and older movies with poor transfer quality. Your results may vary depending on the type of material you watch.