I am using the 2600XT on an old mac, I know it's not the 8800 but it works.
Here is my story...
I purchased my Mac Pro around Oct 06. It came with the x1900, I must have got the POS batch because it starting having the screen artifacts and weird lines and crashes. Anyway it was out of warranty and i didnt have apple care so I went back to PC. I have much man love for PC guy...
When I saw apple were offering cheaper cards as upgrades, I ordered the 2600 XT (I have a mac pro, but I don't do much pro stuff or games so I don't care that it's slow ok...). Btw, in australia the x1900 costs $660AUD/$580USD, um what!
I put the 2600 in this afternoon and it works!.
I was a bit worried the box states: Requires Mac Pro with PCI Exress 2.0 slot and Mac OSX 10.5.1 or later.
So much for that.