I too have experienced frequent freezes my beige G3 since installing a PCI Radeon. I haven't troubleshooted it as far as you have, but as I'm also running a PCI USB card I suspect it's the same.
FWIW, only certain applications (most often IE5, which was always bad but has gotten much worse since the Radeon) seem to trigger it. When I was out of town last week I left my computer on for 5 days running Entourage and Personal Web Sharing and it never crashed, and it's never crashed playing games, which you'd think would trigger the problem if anything would.
I suspect it's a driver issue. I've read the problem is not there in the OSX public beta. *fingers crossed*