I use the Gamecube RGB Scart Cable from GAME (£9.99) which also has stereo phono connectors coming off it allowing direct connection to an amp.
I'm sure it does allow RGB as I have my JVC TV set to RGB on scart 1 and usually if you connect anything that isn't RGB into it you get a black/white picture.
I'm fairly fussy and IMO the picture is perfect and the sound through to my amp is perfect aswell using the GAME cable.
Someone in another discussion said they couldn't find this cable on GAMEs website - I just picked it up in my highstreet GAME store.
Your best option, if you're upgrading to plasma or a projector, would be to get the component cable, and have a bog standard composite cable for the sound (just don't use the video connection). That's what I've got, and it's darn fine!