Win XP PRO and updating to WIN 7.
my computer has only PCI slots. no PCI-E slots,

Upgrading to USB 3.0 my only option that I can see is going the PCMCIA PC Card cardbus to PCI Card Adapter route.
This card allows the use of a PCMPCIA 34mm/54mm express card used in Laptop to accommodate USB3.0
I will have an external 500 GB(SATA) HD with USB 3.0 output to computer. Allowing for faster (10X) backup.

I did locate a 2 Port PCI USB 3.0 card Adapter by Startech but the cost is more than double at this time.
And a PCI to PCI-E converter card is as expensive, and would entail the purchase of a PCI-E USB 3.0 card.

Opinions, suggestions, appreciated.