I remember that full-face helmets (such as the Power Helmet, the Raider helmets, for example) meant nothing else could be on your face (equipped, that is). However, I can accurately confirm that with the Xbox 360 version of Fallout 3, you could wear a Combat Helmet (which just covers your head, and others like it are hats, small helmets, ect) and also wear, say, Sunglasses. I know this because when I have my character wear Combat Armor, I would also put on my Lucky Shades as well, and look dashing.