I have .. sorry i HAD
WoW running on a 2.66 Quad 09 with the ATI 4870.
So i can only speak for a 24 inch Display with 1920x1200 resolution.
All was fine and i say that also as a PC Gamer.
If you also plan to use Bootcam / Windows native i would go with the GTX 285 because it has more Video Ram than the stock ATI 4870.
As an alternative you could flash a PC 4870 with 1 GB of VRAM for your Mac Pro. But this gives the problem, that mostly the second DVI Port will not work anymore so you can use only 1 Screen.
For resolutions above 2500 x XXXX i would go with the Nvidia.
So i can only speak for a 24 inch Display with 1920x1200 resolution.
All was fine and i say that also as a PC Gamer.
If you also plan to use Bootcam / Windows native i would go with the GTX 285 because it has more Video Ram than the stock ATI 4870.
As an alternative you could flash a PC 4870 with 1 GB of VRAM for your Mac Pro. But this gives the problem, that mostly the second DVI Port will not work anymore so you can use only 1 Screen.
For resolutions above 2500 x XXXX i would go with the Nvidia.