If you are finding the DX 11 option is disabled (i.e. you are given a choice between DX9 and DX11, but only 9 is selectable) then you might want to check the properties on Steam.exe. A lot of people say it is set to compatibility mode for Vista. Remove this and DX11 should be available (assuming you have the hardware!)
n.b. You may need to use the "change for all users" button if the check box is greyed out.
Apologies if this has already been posted somewhere.