seems like we're definitely in a very specific minority here... Ive run out of ports on my board, and the only thing I have left available is an open PCI slot.
i tried a very very old SI 3512 card, but it turned out to be a later revision that wouldnt work with the mac firmware flasher. but then again, I dont even know if that card would work on an intel machine.
i also considered looking at HighPoint 15xx series cards which worked in PPC macs too. Being as highpoint has better OSX driver support than most it might work. Ive seen them going for less than $20 on ebay.
I suppose the trick would be to find a SATA controller chip that is known to be working with OSX later versions, and then see if there's a PCI card version of it.