PCI to USB 3.0 card worth it for external HDD with OS on it

I don't really know if this should go in Hard Drive Forum or this one, but here it goes. A while ago I bought a cheap usb 3.0 external hdd on sale. To be exact the Toshiba Canvio 500GB USB 3.0 Portable HDD (http://www.radioshack.com/product/index.jsp?productId=1...). I installed a linux OS (Ubuntu 64-bit) on it and plugged it in my PC's usb 2.0 port since my pc doesn't have a usb 3.0. It seems to me that sometimes the OS is a little slow and was thinking this is probably because its in a usb 2.0 instead of a usb 3.0 port. So my question is would it be worth it to invest some money in buying a pci to usb 3.0 card so I can use the full speed of my external HDD? Would it speed up the OS performance? Finally, if it is worth it any recommendations on good pci to usb 3.0 cards? I have the Gateway DX 4300-22 PC and have 2 PCI slots and 1 PCIe x1 slot open. Thanks in advance.