some G5s have PCI-x slots and some have just plain PCI slots so that depends on which you have. I would image that if it has the plain PCI slots that you could use it as long as the card is OS X compatible.
PCI-X is backward-compatible with PCI, so that's not an issue at all.
The issue is verifying that the card you intend to put in the G5 is a 3.3v card, and not a 5v card -- VERY important if you don't want to fry a PCI slot (or perhaps the motherboard) on your G5.
What kind of card is it? Maybe someone here can look up the specs on it and make sure it's compatible.