Monster Cable GCGL300 SV-10 GameCube S-Video A/V Cable (10 Feet)
Definite Improvements
I had recently purchased a Samsung LTN 325W flat panel television.  As you may/may not know, LCD televisions tend to receive analog signals poorly.  That was no exception when I used the standard composite cable provided with the Gamecube and hooked it up directly to the Samsung.  I decided to give Monster's S-Video cable a try, and I was fortunately not disappointed.  Using an S-Video cable separates the color and brightness of the video signal, allowing it to be processed cleanly and more accurately.  In addition, it allows you to utilize the Gamecube's progressive scan output.  The result, a sharper, more vibrant picture.  Give it a shot with Mario Golf and Legend of Zelda: The Wind Waker.  There were definitely improvements.