If you have downloaded the lates drivers for your video card, rebooted, and you can still not choose 1920x1200 you will need a new video card. Quite simple
What is your max resolution available at the moment?
Please see Samsungs support area
http://ars.samsung.com/customer/usa/jsp/faqs/faqs_view.jsp?isINPUT=Y&SITE_ID=1&PG_ID=0&AT_ID=8476&PROD_SUB_ID=0&PROD_ID=133Which states:
Note: If the native resolution of your monitor exceeds the top resolution setting displayed on the Screen Area slider, it means that the video card in your PC can not produce a resolution that high. To have your monitor display at its native resolution, you will have to get a new video card.