I am glad to announce that I have found a solution to this problem myself with sirprising results.
You were all right. One has to find a way to switch from integrated graphics card to discrete graphic card (in this case the nvidia GT 555M).
for all who have a DELL XPS 17...right click on your desktop...select Nvidia control panel...and go to 3d settings. there you will find a window that allows you to choose between three different options: auto, nvidia and integrated card...selecct the second option and here yoou go.
the graphics card is that good that you can even use shadows...it looks dazzling...i wa sreally impressed...and its not even laggy if you use shadows...i did not expect that given that it was horribly laggy before when the system was using the integrated graphic card..
hope that explanation will help you. and dont return the notebook...its stunning and the graphics card is great for sl.