United suffered a 1-0 defeat at the Etihad Stadium in November, but have since climbed above their neighbours into third following a poor run of form for Manuel Pellegrini's men.
City have lost their last three away matches and Van Gaal is hopeful of inflicting more misery on the champions in the chase for UEFA Champions League qualification.
"I dream of it," he said. "Every player shall dream of victory.
"I've had to think with my staff and players a lot a of days how we have to beat Manchester City.
"You are sitting in that process to win against your next opponent and of course you want to win because it is a big step in the table.
"Third place is then reachable and, a month ago, nobody was thinking about that besides me.
"Third is also good because you are certain to qualify for the Champions League and we are doing better than the goals we set in pre-season."
However, Van Gaal believes two defeats to Swansea City this season will always rank as a bigger disappointment than potentially losing twice to United's local rivals.
"As Manchester United you cannot lose two times to Swansea City," he added. "I think that is a bigger failure than when we lose against Manchester City two times."
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