I've had bad luck with USB2 PCI cards in older Macs (don't work at all). I don't think that any version of OS9 supports USB2. Panther should, and I think that OSX.2.8 does too, but I'm not certain.
I would stick with USB1 in older Macs, and firewire if you need a faster connection (burners or hard drives).
If you really want USB2 I would go with a known Mac card like Macally. The downside is that they tend to be more expensive. I don't have any first hand experience that these will be problem free either.