Use OrangeMicro's USB 2.0 driver
http://www.orangemicro.com/media.html
"Your USB 2.0 PCI card will work fine in OS X without drivers - just at the 1.1 standard.
However, for USB 2.0 compatibility, you need this driver:
USB 2.0
It's an industry-standard driver - all USB 2.0 PCI cards use the same driver under OS X 10.2.x
cheers