I use the Mac Pro 1,1 model and I have heard from many people that the Radeon HD 4870 works fine on these models.

The other option for my computer is to purchase the nVIDIA Quadro FX, but this card costs way too much for me at the moment. Another card, the ATI X1900 XT, I have heard can overheat (yikes!) and has caused several graphical issues that can heavily disrupt productivity.

Anyhow, I have my fingers crossed and hope this card will fix my problem!!