The PS2 has really no progressive scan/HiDef Games to speak of (a few I think?) so you really won't see a huge difference if any over s-video cable, but it will look better then a RCA Video Cable.
on a side note, I am pretty sure that no PS2 games have surround sound in game during play. They say they do but that is only during cutscenes. So unless you use your PS2 as a DVD player digital audio cable is pointless.