Encouraged by directly this discussion, I dug out my quad and reinstalled my X1900GT. Added a CMI based sound card as well.
Have I mentioned how much I hate working on G5s?
I had to replace an SATA cable, which involved pulling all the cards out again and disassembling the hard drive mounts.
This design's layout sucks compared to the easy access a G4 affords to everything.
A real step backward.
Good idea to get it the way you want (no doubt through much pain) and then button it up and leave it be!