I also have a Proformance videocard, on my 9600, and it´s working for me.
first i installed OS X with my original 604ev prosessor installed and it worked, but when i switched over to my G3-upgrade it didn´t...
So i put the Proformance card in PCI slot 1 instead, as i read somewere, and then it worked.
But i can´t get the card to accelerate AQUA, it runs smoother on my Powerbook G3 with has a not so good video card.
If anyone know how to get support for the proformance in X please tell me