Yo. If I'm understanding you correctly, you want to use to the video input on your video card to display the Gamecube signal on your PC. Your video card should have come with some video capture software that will allow you to see the image.
I've done this, and I must say that the quality isn't very good. Unless vid capture technology has improved a lot recently, the image quality probably won't impress you. If you actually have an HDTV, you'd be better served by buying component cables. If you want to attach the GC directly to your PC monitor, there are ways to do it (and the image quality is great if it's done right).
-Angus