You may install one, but it is unclear as to whether or not it will work well.
OS X Leopard and earlier don't have superspeed drivers, nor will they ever have them.
If you want to be a guinea pig and try one, one like this NEC chipset Addonics would be as good a candidate as there is.
It seems that the card might work up to USB 2.0 speeds, but beyond that, and in the absence of PCI-X bandwidth (at a minimum) any gains will be cancelled by lack of bandwidth on the PCI bus.
That is why the majority of all USB 3.0 products are for PCIe systems..... bandwidth.