Dallas was a city in Texas, in the United States of America. President John F. Kennedy was assassinated in Dallas on 22 November 1963. (TV: Rose, PROSE: Who Killed Kennedy) After his murder, Kennedy's body was transported to Parkland Memorial Hospital, also located in Dallas. (PROSE: Shroud of Sorrow)
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map at right is probably a good indicator of the DWU location of Dallas.