What size is the Nikon USB connector?
For an extesion cable I'd recommend getting an active cable. I have a 5m USB mini to A cable and then a 5m active extension cable. That way I was able to have my computer over 30 feet from the camera when I was teathering last weekend. I had D200 RAW images on the laptop in 7 seconds and showing up in Aperture in another 8 seconds. Worked great.