Buy XFX GeForce 8800GT 512MB PVT88PYDF4 , this works perfect as MAC Pro machines are designed with this card. I also had MAC OS X Leopard 10.5.6, it worked fine with this card, played 3D games like Flat Out without any problem.
the cost is five five zero zero shipping paid to you, the PSU you have is good enough to run this card. the card is about 10 months old with all accessories and company of heroes.
The 8800/9600/9800 are recommended cards. AFAIK even thermal management works on the 8800GT. As for the ATI series, they do not have that much support.
My friends 4870 works perfect, but his computer wont wake up from sleep. While my computer (exact same config, though I have GTX260) will sleep and wake-up very well.
the cost is five five zero zero shipping paid to you, the PSU you have is good enough to run this card. the card is about 10 months old with all accessories and company of heroes.
OS X 10.5.7 has support for HD4000 cards... so if you're using that build, you can use HD4000 cards... if not, instead of experimenting with custom Ktexts, it's better you go for 9600/8800 card.