Ehh.
I call bunk on getting decent Core Image support / speed out of a PCI Mac. They are great machines and all, but come on. We are dealing with machines ranging from 33MHz to 66MHz in system bus speeds, most of them hover around 50MH or so (8500's come to mind). Trying to cram PCI Extreme, Core Image, USB / Firewire, SATA or ATA/133 (via add in card, same with USB and Firewire) and run all that through a bus that also has to deal with talking to the CPU, RAM, optical drives, external devices, plus run the OS and Apps is not going to result in decent performance.
On higher end PCI macs such as the 8600/9600, G3's and early (Yikes!) G4's such performance might be possible. I doubt even a super upgraded 7600 could keep pace.
Now I am not saying that over-upgrading machines is a bad thing. I do it all the time, and so do a lot of users here. It's just that from a dollars to performance ratio and factoring in tinker box vs. work box, I wouldn't spend the money on card capable of Core Image over say a Radeon 7000 or even 8500. I would rather spend that money in areas that will have a greater impact on overall system performance such as CPU, RAM, Storage Controller (SATA or ATA/133/100/66), better HD's and so on. In the event that the box was destined to become a dual monitor box, I would buy a card that had 2 outputs. Whether they are VGA or DVI makes no difference as they are easily adapted.
It would seem to me that for an OS X box, trying to round out overall performance should provide a better user experience than focusing on only 1 or 2 areas in this case (CPU, GPU). Having good finder speed or the ability to draw something quickly disappears when waiting for the HD to finish thrashing, or finding out that the slow CPU can't feed the GPU fast enough, thus incurring more bottlenecks.