I'm an admin on /r/gamecollecting on Reddit and we recently made a group purchase of some Monster S-Video cables for SNES/N64/GCN. I wrote a review HERE if you're interested with screenshots comparing composite and S-Video.
Whereas a couple of those cables cost me around 15 bucks, the jump to a top-notch RGB setup (XRGB-3, GCN SCART cable, and euro-to-JAP conversion...) would be 40-50x more expensive. I'm a big enough of a nut that I'm actually open to considering the possibility, but frankly I don't see how the video quality can get much better than what I'm seeing in the screenshots I posted within the review.
Anyone care to chime in? :P