I have the Keyspan USB card; I went to the Keyspan site but did not find any MacOS X drivers for it; I didn't see them in my iDisk, and I have no idea where else I would even look for them. Fortunately, the drivers are still working in Classic, so at least my hardware has not been rendered totally useless.
I have just installed OS X on my beige G3 233. USB PCI Card works fine, even with Printer. Firewire PCI Card shows up in System Profiler, but the FW Harddisk is not mountable. With OS 9 both Cards work well.
I have a Belkin 5U005 PCI usb card and Belkin tells me it should run on OS X and does not need any drivers. I plugged it in and my machine would not start. Belkin says to try a different PCI slot, but I haven't tried that yet. They said the different slot trick often works because each slot uses different resources. I'm not sure I buy this, but it might be something to keep in mind if you have trouble with any card.