You have to check that it is the right version for your mac IE pciX or express - I get confused as to which machine has which and what works with what BUT in my machine, which is a first generation 2.0 dual cpu G5, I have 1X PCI Firewire 800 and 1X PCI firewire 400 cards installed, both work great no problems at all - each one has three PORTS (but ONLY ONE BUS for the three ports this is important to remember don't get fooled by a single bus with lots of ports, still just one bus) - both of these cards are LACIE - I recommend them, they are very cheap, simple, nothing to configure, just stick them in the G5, close the hatch, power up and they're there, simple as that. No problems at all for me. Connected to those I have a number of 800 drives on the 800 card, 400 card has my audio interface and I have another 800 drive plugged into the G5 built in firewire (with no other devices on that bus). All works very well now.
Miklos.
Miklos.