I'm glad you're still planning to run this segment with the various video cards - I'm very interested, as I've got an old AGP 500 MHz machine sitting around.
For my part, I upgraded the original Rage Pro card with the ATI 'original radeon' card - the radeon that was sold retail before they were put in the power macs. It drives a nice big monitor on DVI and supports quartz extreme, but has a very noisy fan and actually feels a bit slower than the Rage Pro for video playback.
I'm eager to see what you come up with.
Building a PowerMac Media Center Part 2: Readying for a Video Card