Simon Helberg (Howard) really does all those impersonations (Al Pacino, Nicolas Cage, Christopher Walken) himself and has done several others throughout the series. These impressions were specifically set up for Helberg because doing imitations used to be part of his comedy act.
The edition of Dungeons & Dragons that the band is playing is the Fourth. While it had its fans, some players felt that part of the radical innovations introduced by it changed too much the basic nature of the game. After many debates on internet fora (and many players switching to other games), it was superseded in 2014 by the very successful Fifth Edition.
Sheldon mentions to Amy that there is a possibility that they might get intimate some day. In The Cooper/Kripke Inversion (2013), he said a similar thing to Leonard and Penny.
In the German synchronized version of the episode, Howard impersonates Leonardo DiCaprio instead of Christopher Walken in one scene. DiCaprio (in movies) and Sheldon are both dubbed by Gerrit Schmidt-Foß. Therefore, Schmidt-Foß also dubbed Howard's version of DiCaprio in this episode.
In The Recombination Hypothesis (2012), Sheldon (Jim Parsons) voiced his disdain for Zachary Quinto as Spock, "Oh, no! They sent the wrong Spock! Live long and suck it, Zachary Quinto." However, we learn here that Sheldon was just upset that he didn't get a Leonard Nimoy cardboard cutout, as Leonard reveals "You [Sheldon] were worried about Zachary Quinto being the new Spock, but you ended up liking him." Parsons and Quinto later appeared on stage together in the play 'The Boys in the Band', as well as the movie adaptation The Boys in the Band (2020).