not great! s-video is best with a high quality cable, scart, or any RGB signal (arcade monitor) kicks it's butt no matter how low the quality of the cable is.
I was aware that the emulaiton scene on PS2 was pretty poor anyway, as Xbox is a much sweeter propostion for chipping and writting/porting homebrew software. I have both a chipped box and PS2 and have to say that xbox is the ultimate emulator. the high resolution output and processing power allow for some highly authentic looking emulation, buy no means perfect but very good, including an excellent MAME emulator and frontend.