When you're watching a movie as file on your computer, you are taxing the processor; if you watch it from a DVD, you're taxing both the processor AND the optical drive. Bottom line: you will have fans running.
Someday, when everyone uses H.264 for video and Apple includes specific decoder chips in their desktops and laptops like they do for the iPad and iPhone, this will be less of an issue.
As for "full screen versus zooming" I could be glib and tell you simply that watching a movie full screen when its not shot in that ratio is stupid, because it is, and leave it at that. No, there's no difference whether the computer zooms it in or you do, it's still zooming in and cutting off part of the picture.
Instead, I will explain that watching a movie at its proper dimensions is important because that way you are a) seeing the entire picture, which is how the film was VERY CAREFULLY composed by the director and cinematographer for very specific reasons and b) getting the full "peripheral" vision of the film even if the action is taking place dead center; it REALLY changes the mood and your brain's reactions to a scene when you zoom in on it versus peripherally viewing the entire tableau.
A perfect example of this can be found by watching the "full screen version" of any of the Lord of the Rings movies. You lose ALL the beauty and majesty (and action) of the composed wider shots. It lacks immersion, the ability to draw you into their world. Compare it to the "wide screen version" and you'll quickly understand what you're missing.
Most Mac screens these days are 16:9 or 16:10 anyway, which is very close to the ratio used by all films, TV shows and HD material shot these days anyway, so the amount of "black bar" should be pretty minimal unless you are a big fan of Cinemascope movies. Even material shot at 4:3 (like older TV shows and very old movies) should be seen with the "vertical bars" rather than zoomed, for the same reason; the picture was composed for a square, trying to stretch into a rectangle will just make everyone look fat and the picture look grainier since you're blowing it up so much.
As I've often told people, "if you are even *noticing* the black bars, you're not really watching THE FILM."
