Seems I'v got one too many USB devices..........is there an add-on or plug-in that allows multiple USB devices to plug to it?

You can get an internal card as seen here:

http://www.usb-2-0.com/usb-2-0-pci-card.html

Or an external Hub as seen here:

http://www.everythingusb.com/usb_hubs.html