if you want 480p output on the gamecube, you must use the component video cable, anything less will only give you 480i. that being said, it is difficult to find cheap component video cables for the gamecube (you also have to make sure your gamecube has the extra port on the back for it, the newer models are lacking this port)
with the price of the Wii just dropping to $199, i'd be looking at picking up a Wii, along with the wii component video cables. THough it is more money than just buying a component cable for your gamecube, it provides you with alot more functionality. there are some really good wii games out there, plus you can play your gamecube games on it. it just seems stupid to me to spend $50 or so on a set of cables for an obsolete game console.