Your graphics are on-board and although they are fairly good might not be good enough.
In which case you would need to purchase a graphics card.
What is your DirectX version? This can be found at:
start/Run/ dxdiag
If it is not 9.0c you can download that here:
http://www.microsoft.com/downloads/d...displaylang=en
What is the exact Dell model and # that you have?