Most PCI cards that do not need to be active on boot (video cards would need to be active at bootup, network cards would not) will be able to function in our machines with the proper software driver loaded. Since the PCI standard is well-defined, there should not be a problem with the hardware.
Open Firmware is required for boot registration services - in other words, in order for the card to be recognized before any OS software loads, a PCI card in our computer will have to be Open Firmware compliant. This would apply to cards such as video and hard drive controller cards.
Specific compatibility questions should be directed to the vendor or manufacturer of the PCI card in question.