Whether you'll get better picture quality from HDMI than from component for the cable box is hard to say - I know that when I've compared the two outputs on both HD boxes I've used, I've come away unsure if there was any real benefit to either connection over the other.

Do you have any other HDMI sources available to test it with? The NS975V should have an HDMI output, if I remember correctly, and if it doesn't work with the SXRD then I'd suspect a hardware fault in the TV.

I've not noticed any difference on Comcast between HDMI and component. Comcast only supplies 720P video anyhow and it's so compressed that there are artifacts (pixellation) every time the scene is changing quickly.

I have DirecTV HD and have both component and HDMI and it is hard if at all possible to notice any difference.

The image quality (audio too) is limited by the weakest link in signal path. Apparently the quality of either the component or HDMI signal transfer between 'box' and display is greater than the weakest link in signal path, which, apparently, is the compression used by the service provider to move the signal through whatever bandwidth alloted for each HD channel.