Kenbo,
While I respect RW's ability to have his opinion, I must greatly disagree. Much will depend on your display, running HDMI to a CRT might not do anything for you, but maintaining an all digital feed to a fixed pixel display (Plasma, DLP, LCD,SXRD,LCoS etc) often will give you significant gains in detail, and I have witnessed this first hand in many systems, both of my main ones included.
As far as cable, yes, there is a great range, and again I must disagree with RW as I have firsthand seen huge differences between different S-Video cables, and have heard plenty of difference in digital cables, I must assume that yes, there is a difference in HDMI cables as well, I have been too busy to demo some of the higher end ones myself, but suspect they would show similar differences.
I know, many of you will bite my head off for this post, as you use cheapo Rat Shack video cables, but I think many will agree with me, albeit silently!
KT