USB2.0 PCI Card slow transfers

Having got the PC stable (finally) by disabling onboard USB completey and going with a USB2 PCI card instead, I now find that the transfer speeds when copying to my external drive are approx 1/2 what they were when connected (same drive.same cable) to the onboard USB2 ports. I have tried 2 different usb cards (both 2.0, one VIA, one noname brand), and the VIA is a little faste than the noname, but still only 50% of the onboard.
Can anyone suggest what the problem is? And if (as i suspect) it is just the PCI cards fault, can anyone recommend a fast PCI USB2 card (needs to work in vista) for my motherboard?