Have you heard of Infernal Affairs? It's the Hong Kong film that the upcoming star-laden The Departed is based on, and it's also damn good. It's about the undercover cop world with a crazy amount of twists and turns throughout. Both The Departed and Infernal Affairs are good films that deserve an audience, but I'm here to plead the case of the original while the subject is still relevant.
My good friend (or at least quasi-buddy) Mr. C. Robert Cargill already tackled the idea of whether or not the American remake should have been made (here), so I'll just take the track of friendly advice. See the original! Oh sure, see the new one too, the cast alone is worth it, but the original has such a different vibe to it. It also features a way more "foreign film" ending that to my mind works much better. The gangster (played by Eric Tsang) actually whups Jack Nicholson's performance, which is no small feat. Infernal Affairs also leaves out an uber cheesy love angle that The Departed feels obligated to give American audiences. If that's not enough reasons then consider that Infernal Affairs won The Golden Horse award, and while I'm not entirely sure what that is, it certainly sounds impressive.
An American region version of Infernal Affairs is out there and cheaply available. I'd also have no problem with recommending seeing it before The Departed, as the films are different enough that you'll still get a new experience out of it. You know what? Let's just make this global advice: If we Yanks remake something, try and see the original. Learning about other countries' styles of filmmaking never hurts, and you'll have more street cred to enamor your friends with.