I saw the test report. The differences noted in the report are for gaming and should not be relevant to image editing tasks. Even the basic card available for the MacPro from Apple are adequate for Photoshop and image editing tasks. More important would be a good measure of RAM (at least 4GB) and a RAID 0 internal drive.
I purchased the first MacPro 8 core that my local dealer delivered in Mar 07. I purchased the middle level card at that time, the ATI Radeon X1900 (512 MB VRam), 12GB RAM and 4 internal 750GB drives. I have 2 of the drives configured for RAID 0 using Apple's system software (no RAID card).
This system screams compared to the Dell Precision 470 workstation that it replaced. Now with Leopard, it runs even better.
There was an early report when the 8-cores started shipping that in real world test for Photoshop and other graphics software, that the 8 core was only marginally faster than a 4 core MacPro. I'm not sure if this is still true now that Leopard is out.
In any case, I think that you find that even the basic level MacPro a considerable speed upgrade over your old Mac.
Enjoy.
Bud James
North Wales, PA