Quality wont change with DVI. Pretty much it will look great or it will be sparkly and messed up. Theres not alot of in between with digital cabling. VGA on the other hand is analog and might degrade.
If you're using a standard 6 foot DVI cable theres pretty much never going to be a problem. When you start to get to 25-100 foot cable runs better gauge cable can help prevent signal degredation.
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