Very unlikely that expensive cables will make any difference at all. HDMI is digital => small amounts of noise in the signal are irrelevant. A bit is either 0 or 1. If a cable is not performing properly, the symptoms would be incorrect pixels on the screen probably in clusters and should be very obvious. I've tried three HDMI cables including gold plated connectors and also DVI / HDMI converters and the picture on a 40" LCD Samsung looks just the same. If you are running very long lengths of HDMI cables there may be a case for looking into more expensive cables.
All this malarky probably stems from the old days of LPs and high quality audio pickups which produce a very low voltage analog signal fed to the audio pre-amp. Even small amounts of corrosion on connectors could adversely affect such a signal, and there is a plausible case for gold plating and high quality cables. Induced 50Hz mains hum was also a factor in mandating good cables. In modern digital systems at much higher voltages, it seems mostly a case of marketing nonsense.