If your Dell Inspiron 660 only has onboard VGA output you can only use one screen. However both Dell Inspiron 660s I have bought recently have a graphics card with one VGA and one DVI output, so ypou can use 2 monitors. If no graphics card then you need to buy one which has two outputs, either VGA, DVI or one of each. I am using a Sapphire HD6450 graphics card which is 48 from Amazon. With 2 monitors, you may experience the same problems as I have, (see first post) but I do not think it is related to the card.