More things to know about high-performance graphics cards:

My old 2-monitor system (the one running XP with a graphics card that can drive 2 monitors) was playing up giving all sorts of odd (intermittent) effects including switching monitor 2 for monitor 1 and losing the desktop.  The card is fitted in one of these cases that doesn’t need screws – the card is held down by a plastic lever and the electrical connection to the case chassis through the metal end plate of the card was poor to say the least.  I used a screw to firmly fix the card to the chassis and all the intermittent problems have disappeared – so I guess that on these higher power cards (the ones that use an extra power connection on the card itself) there is a significant return current to the case  chassis from the metal end-plate of the graphics card.