It's a bit complicated, it will only look good if the game supports widescreen resolutions, if it doesn't you can go to your NVidia driver settings(I guess ATI/AMD has it too but I don't know their drivers), widescreen options and then tell it to use a fixed aspect or whatever it's called, there are two options, one that keeps the aspect and stretches it so it fits the screen in one direction so you get black bars only on two sides and the other is basically 1:1 pixel mapping so you get black bars on all four sides depending on the resolution.

now some screens can also do this, one reason I settled for the Benq 24" is that it does it pretty well so it also works with the PS3, someother monitors like the Samsung SM224B or so have their problems with that. It might help to search google for tests and user experiences to find out about it as manufacturers will hardly say that their pixel mapping doesn't really work.