You're a paragon of virtue today. In fact, you don't even glance sideways at the burgers and pizza as you make a beeline for the salad bar. Sure, you know not to drown your single leaf of lettuce in a sea of blue-cheese dressing, but did you know that other nutritional landmines lurk amongst the dark leafy greens? We asked Bettye Nowlin, R.D., a Los Angeles-based spokesperson for the American Dietetic Association, to take us on a guided tour of a typical salad bar and rate the roughage. KEY: ++++: Dietary powerhouse +++: Indulge with impunity ++: Nutritional lightweight +: Overindulging…