If your going for realism, check these films out:
Memphis Belle - Doesn't accurately follow the last mission of the real Memphis Belle, but it shows what truely did happen on B17 missions in 1943.
Saving Private Ryan - Got some pretty realistic ground combat here.
Hamburger Hill Follows the story of the 101st Airborne as they attack a fortified hill in the Au Shau Valley in Vietnam. True story.
Platoon - Probably the most famous movie about Vietnam.
The Boys of Company C -Another good Vietnam movie.