FYI, I grounded my three RGB grounds to a common ground. There was a noticeable improvement in image quality.
Before grounding, I had noticeable "ghosting" (capacitative ripple) beside every vertical line.
After, I still had some ghosting, but it was far less visible.
Ground lines are your friend. If I had room in my case (and I don't, 'cuz I did something kinda stupid when I did my first VGA-out mod), I'd also consider a ferrite bead on either end of the VGA ribbon cable.