Heck if you have a spare pci slot then just get a USb 2 card, they are cheap.
http://www.ebuyer.com/product/192862
If you have a spare usb header then you can use a pci slot cover with usb sockets that pulgs into the usb header like this.
http://www.ebuyer.com/product/119520
If you need front access then you can use a simple usb extension cable.
Although I think my biggest question is how have you run out?
ahh just looked at the ASRock Dual Sata2 939, only 4 rear ports and 2 internal headers (each header supports 2 usb sockets.)
Still what are you using that's takeing up 4 usb solts?