hmm i wouldnt go about upgrading your pc just yet without first trying to see if you can get USB 2.0 to work, what USB card are you using? is it straight into a usb header onto the motherboard or is it a PCI usb card? Does it need drivers? A Bios update may help, but only if the USB card is plugged straight into a header on the motherboard, if it is a PCI usb card, shouldnt make any difference.
A good way to test to see if this really is slowing down the read times from your external disk would be to check file transfer speed of downloading a large amount of music with your music server by using a program called ATTO (http://kb.ciprico.com/lore/article.php?id=268
then do the same thing with the same amount of large music with your newer pc and compare the times
If you could give your motherboard details and more about the music server that would be useful