Hey, thanks to all for the constructive comments.
A few details about the system:
Intel board with 955 chipset. 4GB RAM. XP Pro 32-bit with all Service Packs except SP3 due to some potential conflicts with BeyondTV. 840 series Pentium at 2.8 GHz. I have a 3.2GHz 940 that I can put in, but was not going to if not needed. I don't have any stuttering or playback issues, so I'm not too concerned there. Also, I do have the Platinum-level Nvidia PureView software decoder I bought a couple of years ago installed. I don't know if the card augments that or overrides it or works with it. Cyberlink PowerDVD shows that I am using PureVideo decoding in its config window. The specific card is a Foxconn 8600GTS.
I have considered ATI and actually have a spare HD 3650, but since I only have one graphic card in the system I was not overly concerned about the power draw.
Maybe it's subjective, but the Cyberlink player seems to do a better job that WinDVD 9+BR. I would say neither hits the quality of our PS3 player. Both the PS3 and the HTPC feed via HDMI.
If this changes anyone's opinion I'd be interested in knowing. If upping the CPU can help, I will do that. If upping the card would make a difference, I would do that. It is best for my situation to have a half-length card. A full-length card is too tight.
I am open to ATI or Nvidia if a card upgrade would make a substantial difference.
Thanks for any ideas.