You can choose any PCIe x 1 add-in desktop adapter from the list below. Make sure to read the product documentation before purchasing them.
If you are not using any ide devices and the onboard lan, try to disable the onboard lan in the bios. It may help as these two devices sit on the link that the pcie connector is connected.
See TPS Page 14/90 Figure 2.
Just a quick note, Windows 7 is not a validated OS for this board, you may need to run the vista driver in compatibility mode.
Let us know how it goes.
All the best,