If I understand you correctly, you're looking for a PCI card that has speaker terminals on the back to which you can directly attach you Axioms, right? As far as I know, no such product exists.
If you have a Mac G4 with a digital (non-optical) audio out connector, you can purchase
this breakout cable which allows you to connect directly to any 8ohm speaker.
What I would do in your situation is buy the
T-amp that everyone's been talking about. It's cheap, small, and sounds great. Just connect it to your line out with a standard mini-stereo to mini-stereo plug (included), then connect your Axioms to the spring-clip terminals. The t-amp has a volume knob on the front for easy adjustment.
