there are a few diffrent versions of PCI extreme. there is version 2.x and then there is version 3.x. the 2.x didnt work well on tiger and 3.x worked fine (for me anyway). you should try both versions of it.

if you are trying 2.x then you should try 3.x , if you are trying 3.x you should try 2.x

there is sometimes PCI extreme just will not enable, and sometimes it will enable but will hose the install to where you cant boot OS X up. or it will just work.

if you have tried both versions and its still not working. try installing tiger on it and then trying it.

when you open PCI extreme to enable it you press PCI and a dialog box pop's up saying it will open a terminal window and you have to enter your user password so it can edit a system file. you might not see anything being written down but it is and once its done it will reboot or ask you to reboot.

on tiger it has always worked when i enabled it. most Mac's are not the same when it comes to getting some stuff to work on them. have you seen if permissions need to be repaired before you try to enable it.

just trying to see what is happening or what isn't happening. there has to be a reason its not allowing to be enabled.