Bought a Sonnet Tango 2.0 : it was not cheap, but it was new! Nice purple card, works great!
Previously tried an Adaptec DuoConnect AUA-3020, but it was only half working (the FireWire half). And prior to that I had bought (real cheap) a Belkin F5U508, but with a VIA chipset there was no way it would work! (narrator: it didn’t work at all)
So, the Sonnet Tango 2.0 is the way to go for an hassle free USB & FireWire upgrade for the 6400!