No S Video or RGB can be gotten from NES systems. To try and do so is to go mad. Trust me, I put about 120 man-hours of research into it, to no avail. The PPU outputs NTSC video right from the leg of the chip. Trying to get it to function in RGB would mean:

1. Building an expensive PCB with expensive chips and pots to separate the color and sync signals. By the time the signal goes through all those chips, the picture quality you hope to gain is gone.

2. Getting the RGB/SCART outputting PPU out of a Famicom/TV combo unit. Good luck finding one, and if you do, WHY BOTHER GUTTING IT FOR A CHIP WHEN YOU CAN USE AN NES ADAPTER ON IT AND PLAY GAMES IN RGB AS IT IS?

3. Getting an RGB outputting PPU from an Nintendo Playchoice. These don't exactly grow on trees, as we all know.

Much simpler method would be to just mod your Dreamcast for RGB and use the NES emulator, as I ended up doing.