Well, first, through the S video out, you aren't getting HD. S can only handle 480. Is your Gateway an EDTV? If so, you are only seeing 480 from any channel. In any case, SD channels will look anywhere from decent to crap on a large screen display. Garbage in-garbage out.

The only way to make this better, and it may or may not work, is to add a scaler to your system. The scaler will take whatever signal is input and convert it to whatever resolution you desire as output (use the native res of the panel) It MAY give you a better picture on the SD channels. If the source (cable box) is sending a crappy signal, it will still look crappy. Try the ISCAN scaler.

Utilize the DVI if you can. Again, it should provide a great picture for the HD channels, and possibly a better picture on SD channels.