The USB can be quite fast if you use a USB3 adapter which your machine would not have. I doubt by the vintage of your machine's cpu you'd notice much difference between the two interfaces. But I suppose every little mb/s faster counts.

What is the chipset of the Sata card? What are the specs of the computer. Is it possible you could have a dead card. Is it brand new? Bonus dumb question - Is the card seated in the PCI slot correctly?
