This varies greatly also with version of Mac OS.
In general, any OHCI-compliant PCI card will "just work." In my original 7300 workstation, I bought a $9 Craptastic-brand USB PCI card from CompSpewSA (when they were still in business) and stuck that in, and it worked fine in 9.1. I can't see why that would be different for the 6500 specifically.
Generally I recommend the Sonnet Tempo Trio as a great all-in-one card, but they are getting rare and expensive, and the PCI bridging issue rules it out for your purposes.
In general, any OHCI-compliant PCI card will "just work." In my original 7300 workstation, I bought a $9 Craptastic-brand USB PCI card from CompSpewSA (when they were still in business) and stuck that in, and it worked fine in 9.1. I can't see why that would be different for the 6500 specifically.
Generally I recommend the Sonnet Tempo Trio as a great all-in-one card, but they are getting rare and expensive, and the PCI bridging issue rules it out for your purposes.