I finally saw the American remake of Infernal Affairs and it mirrored the original film to a certain extent. It would appear that the screenwriter used the original script and added a bit more sex, violence and random characters to make it slightly more interesting to those who have seen the original trilogy. The following are notable differences from the original:
Instead of Thai drug dealers, Costello (Sam) works with Triads, who speak really poor Cantonese, to smuggle microprocessors into China and gets away with it unlike the original film. Additionally, the two moles use text messaging to exchange information instead of Morse code and the cops fail to arrest Costello because of idiotic bugging instead of destroyed evidence in the original.
Mark Wahlberg is great as Bryce Dignam, whose equivalent character was dead at the start of Infernal Affairs. Not only is Dignam vulgar, he is the one who eventually kills off Sullivan (Lau) at the end of the movie.
They had the psychiatrist in a love triangle between to two main characters. Other than that, The Departed was an ok movie.