Long since given up
I eventually just decided I could have my cake and eat it too, and bought a triple-head Parhelia, put it on AGP and sold off the nVidia cards.
The PCI bus problem isn't really nVidia's fault (it looks like most PCI video drivers have this problem, and I suspect Athlon chipsets make it worse), but I too wish there had been some way to more intelligently handle PCI video writes than 'screw the rest of the system!'
What really eventually killed nVidia in the deal was the XFree86 driver crashing three times a day. I've had the Parhelia running for a month without a glitch now, and I have accel across all three heads.