If it is a USB 2.0 card, it probably wont work. You need a true 1.1 card.

Apparantly, 2.0 cards work fine in BW G3 or later models. They also work in Beige G3s but only if it has a $77D.45F1 rom. Anything older requires a USB 1.1 card.

I have got a feeling this has to do with the PCI specification the card supports. Most usb 2.0 cards require slots that support at least PCI spec 2.1. Older macs only have slots that support PCI 2.0 spec which is what most usb 1.1 cards require.