HDMI cable made a difference in picture quality??

I just bought an a 42 inch LG LCD flat screen TV which is has full HD function. I have a HD cable box and opted to use RCA cables betwwen the box and TV. I am getting missed opinions from the cable provider and the LG dealer about the usefulness of the HDMI cable hookup. Does it actually improve picture quality??? If so,how much? and will it also improve non-HD programming??? I have seen several adds from Monster Cable and Audioquest touting their HDMI cables. Monster has one that is about $70; another is about $100. I have seen even higher price tags from Audioquest. All comments welcomed. Thanks Jim
Toufu, if that were the true then the Wegrzyn HDMI cable would look the same too...faded and missing details .. ???
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Hifisoundguy, it would not be likely for digital artifacts to show up as faded or missing details.

For analog tranmission, yes, since the output of analog is a direct product of the signal being transmitted and if cable is suspect, then result picture/sound can appear as if there were filter applied to make the picture look distorted, faded, etc.

But for digital, the analog is first encoded to digital and then the digital bits are converted to analog to transmit the signal, then the analog signal gets picked up and converted back to digital and finally to analog again. So any noise in the transmission would not appear uniform such as that the noise introduced fades the picture.

Think about back in the days of analog TV signal, if you don't have clear signal, your picture looks distorted, but at least in a recongnizable way. But if you have an HD antaena to pick up digital signal, you either get a picture or you don't. Or if it locks on a weak signal, you can see the digital artifacts very clearly (blocks of green or bad pixels).
My 2 cents is try to find out if the cable you're interested in has a good reputation for staying in the HDMI port. The HDMI connector is inherently flawed in that it is a small plug that relies on friction to stay in. Some HDMI cable connectors simply fall out of the port, particularly if the cable is stiff or heavy. I had bad luck with Audioquest in this regard. If you get the cable home and it doesn't make a tight connection or risks coming out of the port, get another cable.
On my TV the HDMI cable was much worse than the component cable. That said the HDMI cable was a freebie from the cable company. Also maybe there was something wrong with the HDMI output on my cable box. I would try both and use which ever is better.