You need something like
this. Not RGB, just composite, which is what the N64 outputs. Although those ones are used, I'd go for a used official cable over a new cheapo one, as in my experience the official ones were better-made.
Anyway, I don't know if you realised that the only way to get stereo sound from the N64 is to use an AV cable; the coax only carries mono. If you've got a surround system you can connect the audio outputs directly to that instead of the TV (they can be disconnected from the SCART adapter) and you can enjoy surround sound on those games that have it.
As far as s-video goes, the resulting picture is generally too bright to be useful without modifying the cable (I tried it), so the composite is going to be the best that you'll get without resorting to soldering a resistor into the connector plug. In any case the composite picture will be a good improvement over the coaxial one .