Setting widescreen to true will get rid of the black bars, but the game is stretched vertically, that is all persons are now super-thin.
Here's the trick to get the right proportions if your monitor has a 16:9 aspect ratio.
Edit the config file mydefault.ini in Advent Rising's system folder and find the following two lines:
Set the value of FullscreenViewportY to your maximum vertical resolution of your monitor, e.g. 1080.
Set the value of FullscreenViewportX not to your maximum horizontal resolution but to vertical resolution times 1.3333333, e.g. 1440.
(If your monitor has a 16:10 aspect ratio, eg. 1920 * 1200 it will still be a bit off.)
Now the picture gets stretched to it's correct proportions by your monitor. Sorta like with anamorphous DVDs.