GRILL DIRECTIONS: In small bowl, combine all dressing ingredients; blend well. Refrigerate until serving time.
Heat grill. To flatten each chicken breast half, place, boned side up, between 2 pieces of plastic wrap or waxed paper. Working from center, gently pound chicken with flat side of meat mallet or rolling pin until about 1/4 inch thick; remove wrap. Repeat with remaining chicken breast halves. Sprinkle chicken with chili powder and garlic powder.
When ready to grill, place chicken on gas grill over medium heat or on charcoal grill 4 to 6 inches from medium-high coals. Cook 8 to 10 minutes or until chicken is fork-tender and juices run clear, turning once.
To serve, arrange lettuce, tomatoes and cheese on 4 individual plates. Cut chicken crosswise into slices; place over lettuce mixture. Arrange tortilla chips around edge of each plate. Spoon dressing over each salad. Top with sour cream.