I came across this for the PS2, and it looks like there's a similar "D terminal" cable for the Gamecube. It's basically a VGA + audio cable that plugs into the PS2. It does not do any scaling or deinterlacing, so it's taking the 480P or VGA output directly from the console without modification- a good thing!
http://www.playstationmodchip.com/pr...ls.asp?ID=1048
I haven't tried one of these yet. I'm very tempted to do so, especially since I had such a positive experience with a VGA box for my Dreamcast. However, you should be very careful with the so called universal VGA boxes. I tried one that took any s-video or component output and converted it to VGA, and the results were hideous.