Here goes my real life experience...

We have 3 Catalyst systems, all with G5's 2.0 DUAL Processors.
We normaly run up to 4 Videos (diferent Layers) at the same time, and pictures or masks on the other layers if we want to.

The speed process, we do not sense in the processor, but in the Hard Drive access, we have in each our systems a 4 HD RAID 0 setup, on the FW800 port, that gives us up to 90 Mb/s (real). Using movies on the DV codec, we are able to run 4 videos in diferent layers at the same time, after than that the videos get SLOW and Jumpy.
We also figure aout, if we put a 5th Video in the Internal HD, also runs OK.
But all depends of the compression of the Videos and the Codecs.
Without compression, we are able to run only 1 Video without Slowing.
In ANIMATION codec, not even 1 plays fine.

So the processors does not seem the slow problem, the Video Codecs and the HD access seems more hard to solve than the G5's itself.

BTW I did tried some HD content, and i was able to play 2 HD with DV codecs. And the rate was arround 28 frames/s acceptable.

Hope that helps.
Gian