S video only will pass a 480I (interlaced) signal meaning there is 240 vertical and 240 horizontal and where they meet that interlaced, your cable connection with red, blue and green (component) will pass signal of 480P (progressive) 720P, up to 1080I depending on how high your monitor will output, but even if its 480P max that means you will have 480 pure lines of horizontal resolution and no lines will intersect causing distortion. Make sure your DVD player and cable box is set to the matching progressive setting for best picture. The Plasma should be 720P capable but thats a guess on my part.
I know you didnt ask but thought this info would benefit you......cheers