Hmmm it seems people fail to realise what pixel-doubling is. If you run at 800x600 with pixel-doubling on, AVS is simply running at 400x300 and scaling it up 200%. It's useful when you're running in windowed mode and want to see AVS in a bigger window, or when your video card doesn't support 32-bit resolutions lower than 640x480, although I don't think anyone has that now.
I guess it would also make a different if you use overlay mode. Your video card uses a smooth scaling algorithm for the overlay, but AVS uses a pixelized zoom for the pixel-doubling. So in regular overlay, you see a smoothed image. In pixel-doubled overlay, you're seeing a smoothed blocky image.