At the moment I am using a no name realtek 8139 PCI ethernet card and a no name firewire PCI card. In the past I've also used a sil3132 pcie raid controller. All work exactly as expected (cept the FW400 is fast enough on the slow PCI bus). There are loads of chaps out there using rocketraid pcie controllers also - the company has actually written the kexts to power the cards.

So the upshot - PCI/PCI-E buses in our hacks obviously do work (nothing gets reported in sys profiler) its a matter of getting the cards that are either supported by OSX by default (generic Firwire800/400/USB) or cards where the manufactorer has written the drivers.

Unfortunately something like a wireless card is not really going to work because no one would bother writing drivers cause apple already has airport. But I'm still pretty sure you can get wirless PCI card to work with a bit of muffing around.