It really shouldn't make a visible difference at a normal cable length. Analog degrades faster but that only matter with long cables. You can't find a VGA to DVI-D adapter because the D stands for digital. Only DVI-I can transport an analog signal. From what I understand, you have a DVI port on your monitor and a DVI port on your video card, so why don't you just buy a cheap DVI cable to connect them without an adapter?