The Cottagers hammered their West London rivals from pillar to post with Andy Johnson netting a hat-trick whilst USA international Dempsey scored late and set one up for Bobby Zamora with skipper Danny Murphy converting an earlier penalty.
Dempsey said it was vital for Fulham to grab an early goal in front of their home fans and feels that Johnson's 2nd minute finish set the tone for the morale-boosting victory.
"Anytime you score goals it's going to give your team confidence," Dempsey told the club's official website.
"Getting the first goal of the day, it settles you a little bit and we were able to keep possession and that's important considering how hot it was on Sunday.
"I think they [QPR] got a little bit tired. We were able to stay compact at the back and make it difficult for them to create chances.
"I'm sure they were getting frustrated but we were patient and picking out our passes on Sunday.
"Sometimes you play really well but can't score and other days it seems like everything is going in, and Sunday was one of those days."