I use the M-Audio Revolution 5.1 PCI card in my Dual 1.8 GHz G5 with FCP 5.1.1 and once you set up the output channels within your timeline (and in sequence settings) it works just fine.
You need to use the Beta 1.1.1 driver with this card as they don’t have a driver specific to OS X 10.4.6.
Also export OMF from FCP will take you audio out as it appears iin your timeline to an external 5.1 editor (I use Bias Deck 3.5.3).