Its Infernal Affairs, the department which Andy Lau (Triad mole in the police) works in InTernal Affairs, a department of the police force. It was just a play on words, the real title in Chinese is completely different and refers to buddism.
The R33 only appears for like, 2 minutes, it's cool though, the Triads should have been shown using them more hehe.
As for the actors... LOADS. Same two directors. Here we go, Initial D on the left, IA on the right. I wont bother with names probably cos no one would recognise em!
Leader of the Redsuns = Young Triad Mole (Appeared briefly in I and II)
Leader of the Nightkids = Young Police Mole (Same as above)
Main characters best mate = Triad who dies in the car with face on the horn (IA 1)
Main characters dad = Inspector Wong (Guy who landed on the taxi)
EDIT: Oh, as far as I know, IA III was never properly released in the UK, so you might have to rent/ buy it from a place which does imports. Or you could also buy the trilogy online. Play.com has it I think, but the box art is SHOCKING. Does the series no justice.