USB Card provides a simple and affordable way to add two Universal Serial Bus (USB) ports to your Mac or PC.With this USB upgrade it is easy to connect your computer to the USB printers, joysticks, scanners, disk drives and more without the need to disconnect one USB device to another since you will have additional ports to put in your USB devices. A USB card composes of a USB host controller and several USB ports that allow you to install a wide range of USB peripherals. USB cards come in two favors: USB PCI card, and USB CardBus. The latter is only made for fairly old laptop computers. USB PCI cards are designed solely for desktop computers. Simply put, USB cards are devices that let you add another USB Port, normally, 2 USB Ports in one USB Card.

claire_89,

For comparative prices see this.

Depending on how many pci slots are left in your computer
and if it already has a Firewire Port, you may want to look
at Belking USB / Firewire combo cards

see this url:

http://catalog.belkin.com/IWCatProductPage.process?Merchant_Id=&Section_Id=200433&pcount=&Product_Id=125578