Hi, I'd just like to preface this by saying that I'm an electrical engineer, and though I graduated from SDSU I do in fact know what I'm talking about.
Expensive HDMI cables are of no more value than cheap ones, the entire idea behind HDMI and DVI for that matter is to eliminate the need for gold plating and dielectrically or any other type of insulated cables. If you are running component or S-video or even composite, the quality of your cable may, and I mean MAY, be noticeable. The material the cables are made of (Aluminum, Copper, Gold) is hardly noticeable in anything but very long runs and/or the absolute most sensitive equipment, and gold plated connectors are only good if you have loose connections and you need something that will mold a bit more than copper or aluminum, or if you are working with something that will be outdoors or exposed to salt-spray sea air.
If you wanna spend some money and make your systems look better, and give monster cable some money, go and get yourself a power conditioner, they are the the things that look like really overpriced outlet splitter. They will make a difference, especially in audio equipment. The power you get from your power company is usually pretty noisy and will be significantly off of the 110V 60Hz the equipment is expecting, most TV's and Audio amps now have decent power regulation, but if they don't the after-market line regulators will help improve the life of your equipment as well as possibly improve the quality of the image and sound.