In small bowl, mix sour cream, salsa and 2 teaspoons of the taco seasoning mix; refrigerate while making salad.
In 10-inch nonstick skillet, cook beef over medium-high heat 5 to 7 minutes, stirring frequently, until thoroughly cooked; drain. Stir in remaining taco seasoning mix and water called for on seasoning mix package. Cook as directed on seasoning mix package.
Divide lettuce among 6 dinner plates. Top each with beef, beans, cheese and tomato. Drizzle with dressing. Serve with tortilla chips.