LCD TV Help

[EAB1977]

I am helping my parents buy a flat pannel HDTV for the first time and
even I am having some difficulty helping them out.

We went to the local Best Buy store and looked at the 52" LCD TVs. We
were talking to the sales person about the HDMI cables that came built
in to the TV. When we asked about the HDMI cables, she stated that we
NEEDED this to get the TV working correctly. My father recently
purchased a nice Harmon Karmon surround sound unit quite a while back
and I don't remember seeing any HDMI hookups for this (this could have
been before this technology was out). All that is available is the
cable that runs our DirectTV (which we have no HD channels), a DVD
player (non-HD or BlueRay) and a Harmon Karmon surround sound unit,
all which use the yellow/red/while RCA cables.

So, my question is, do I really need to spend the extra money on HDMI
cables when I don't even have the hookups for them? Can I still get
the High Definition TV using the RCA cables? What about over the air
channels using a antenna?

If you could, please be through with your answers! Thanks!